As the second question has the shorter answer, let us address it first.

Entrope was picked primarily by accident.  It was originally colonized by the pre-Maghiz Daryens because of its vast mineral potential.  Plus, unlike most mineral rich worlds, it actually had lots of water (in the form of its vast ice oceans) and an atmosphere (though a very thin one).  So, while the colonists couldn't live "native" on Entrope, its environment was easily harnessed.

The Daryens always intended to fully exploit Winston, but the Maghiz occurred before any real attempts were made to colonize it.  And when the Maghiz hit, Entrope had no where near the resources necessary to transplant its population.  They were pretty much stuck where they were.  By the time Entrope resumed interstellar travel, they were pretty well content with their situation, and Winston eventually became only a farming colony.

To answer the first question requires more explanation.  To properly address it requires a broad look at Entrope's history, so let us just begin.

As mentioned above, Entrope was colonized by the pre-Maghiz Daryens as a mining colony.  Entrope was a rather unique mining colony, however.  Despite the fact that pressure domes were necessary for the population to live, the environment was remarkably forgiving.

In a vacuum, a break in a pressure dome usually means death to most occupants.  On Entrope, it meant that pressure was reduced to unpleasant levels until repairs could be made.

Also, despite its frozen surface, agriculture was easily possible under the pressure domes.  The soil was organically friendly, and the frozen seas provided vast quantities of ready water.  Entrope was agriculturally self sufficient, and not just with algae foodstuffs.

Entrope was also spared the direct effects of the Maghiz as they were just outside the area of effect.  Indirectly, however, the effects were devastating none the less.  Entrope was almost immediately lost contact with Daryen and the other colonies.  A single bulk ore carrier, the Nilozgas, came fleeing the effects of the Maghiz and reported the news of the disaster to the people of Entrope.

The Nilozgas ended up being the only ship Entrope had available to them.  After a brief debate, it was decided to use the Nilozgas to see if there were any other survivors in the remote colonies, and rescue those who needed rescue.

The Nilozgas was immediately sent off to the prison colony of Torment, as it was not known if they could survive their incredibly hostile environment without regular imports from Daryen.  The prisoners were shocked; both that the Maghiz had occurred, and that the people of Entrope would rescue them.  Stunned by the display, most of the prisoners took advantage of their second chance.  They were still kept segregated, but given opportunity to prove reform.  A surprising percentage were able to prove reform.

The Nilozgas was then quickly refitted to allow transport of people.  It then began to proceed down the main and bring back those who wished to move to Entrope.  The entire population of Sehkehm (now known as the Sword World Tizon) was brought back to Entrope.  The Nilozgas then set out to Cunnonic to check on its population.  It was never heard from again.

Entrope was now completely alone.  They had no interstellar capability, a population of almost 100,000, a couple hundred hardened criminals and a few thousand refugees.

They immediately set out to recover the lost Daryen technology.  They had many records and technical information, but there were vast gaps in the information.  First, they focused on environmental sustainability.

It took around a century to discover a sustainable technology for their pressure domes.  The old pure Daryen technology was built to last, but even so, parts wear out, and one of the three domed cities had to be abandoned so that it could be cannibalized for parts to sustain the other two cities.  Once they had discovered sustainable environmental technology, that third city was used to test it.  Once the technology was proven, it was used to rebuilt the first two cities.

Now that they had a sustainable environmental technology many reproduction control laws were repealed.  This allowed Entrope's population to gradually expand.  By the time Entrope was recontacted by Mire they had a population of a couple hundred million.

There was great celebration in Entrope when a Mire Recontact Team made contact with Entrope.  The team was treated like celebrity, and the information they provided was taken in with great enthusiasm.

Entrope had progressed to around TL 8 and was busy reexploring their own solar system when the Recontact Team arrived.  In a little less than 50 years after that they were at TL 9 and building jump capable ships.

Since Entrope was so far from the Daryen Group, it effectively operated as an independent state with respect to the reconstituted Confederation.  Unlike the other distant surviving colony, Zamine, which operated as a client state to the Daryen Group, Entrope took the stance of a friendly independent.  It took full advantage of this freedom and enthusiastically traded with the Zhodani and various Sword Worlds, in addition to the Daryen Group.

A surprising result of this independent stance was the slow, but steady, stream of immigrants from various Daryen Group worlds.  Mire ran the Group with a rather heavy hand, causing many to be disaffected.  Some of the most offended immigrated to Zamine and Entrope.  (Some claimed they were exiled, but officially they were listed as immigrants.)  Another surprise were the occasional Sword Worlder group that chose to emigrate to Entrope.  The Sword Worlds were, as always, a very volatile place, and many on the losing end of an argument decided that voluntary exile was preferable to the alternative.

Entrope also uses this time to create its own little niche.  It creates an agricultural colony on Winston and mining colonies on Anselhome and Torment.  By the time of the founding of the Imperium, Entrope has its own little empire of four colony worlds.  It has reached TL B, has a population of just over a billion people, and is a minor military power.  Entrope is entering its golden age.

Entrope is very careful to choose a neutral path.  While they are on friendly terms with the Daryen Group, they also work to build relations with both the various Sword World governments and the Zhodani.  They are also careful to make sure everyone understands that they are only interested in their world and their three colonies and that they are not interested in expanding further.  This works well for over two centuries.

In Year 104, the Sword Worlds unify into a single government called the Triple Dominion.  The Entrope is careful to cultivate a good relationship with them, which works for a hundred years.  Unfortunately, by Year 200 the Triple Dominion is in trouble.  There is significant disenchantment with the government, and it is in danger of falling to pieces.  The Triple Dominion decides that to survive they need some kind of victory.  They decide that Entrope would be a juicy target and launch a massive preemptive attack.

Entrope is caught completely by surprise.  The mining colonies on Torment and Anselhome are completely destroyed.  The colony on Winston is sacked, though most of the colonists are able to escape into the outback.  The attack on Entrope itself is initially successful and troops are landed on Entrope.  Its military base is ravaged, and its highport is destroyed.  Once the Entropic fleet regrouped, the attack is finally driven off and the stranded ground troops are either killed or captured.

In the aftermath of the attacks, the Triple Dominion eventually falls because of the unsuccessful invasion.  While they could probably win with a sustained attack, they were completely counting on a sudden victory, which they were denied.  Entrope is forced to ask the Daryen Group for assistance and, within the next two decades, formally joins the Group.

The sudden attack by the Sword Worlds did more than end the golden age of Entrope.  It broke the spirit of the people.  The forced realization that their independence had only been an illusion was almost too devastating to face.  Entrope spent the next three and a half centuries trying to understand what happened to them.  During this time they were an amazingly placid member of the Daryen Group come Darrian Confederation.

This placid existence was again shattered by the Sword Worlds.  During the turmoil of the First Frontier War, the Sword Worlds reunited again as the Second Dominate and operated as Zhodani allies.  Sensing weakness in the Darrian Confederation, they attacked and captured the four Entropic Worlds.  This government was careful to only attack Darrian installations, and left Entrope relatively unscathed.  The Second Dominate decided to use diplomacy in addition to just military might to sway Entrope to their side.

Entrope, while a member of the Darrian Confederation, had never really been part of it.  The Darrians had helped rebuild Entrope and its starport, but never included them fully in the defense structure.  The military base was never rebuilt or replaced.  The Second Dominate offered to include them as a full Dominate member.  No Sword World government had ever made this offer to a non Sword World before.

As a result, Entrope's starport was improved and the Second Dominate helped Entrope build a naval base.  During the Second Frontier War, Entrope was a full and enthusiastic member of the Second Dominate.  Unfortunately, the Second Frontier War went badly for the Sword Worlds, and several of the Sword World planets were occupied by the Imperium.  While Entrope was not occupied, it was forced to dismantle its military base.

At this point Entrope finally let go of the idea of being a military power.  They had tried it twice, and both times failed badly.  They were also tiring of the constant infighting and competition between the various Sword Worlds which had escalated after the Gram Coalition replaced the Second Dominate.  They had joined the Second Dominate and wanted nothing to do with the unstable Gram Coalition.  So, secret delegations were sent to the Darrian Confederation.  They let known that they wanted out of the Coalition, and were willing to rejoin the Confederation to accomplish that.  They did demand to be a more active member this time around.  The Confederation agreed.

In Year 788 the Darrian Confederation attacked the Gram Coalition and drove their forces from the Entropic worlds.  The Coalition was not prepared for a war, and was forced to quickly back down.  (This also caused the Gram Coalition to be overthrown by a new government.)

At this point the people of Entrope seem to have accepted what they believe is their place in the universe.  They are an important part of the Darrian Confederation.  They have a large voice in the running of the Confederation.  They are a large and successful world, with over ten billion people and trade relationships with the Imperium, independent worlds, Zhodani and even some of the Sword Worlds.  This situation lasts for nearly three centuries.

The Third Frontier War had no impact on this situation, but the Fourth did.  During the Forth Frontier War, Entrope was savagely attacked by the Sword World Confederation.  The Sword World Confederation does not use the Dominate's diplomatic approach, not trusting the Entropic government.  They bomb the starport, completely destroying it so that it can't be used militarily.  They also cause the government to be overthrown and replace it with a dictatorship by a popular stooge.  The Entropic worlds (which does not include Torment anymore, as it is again an exile world) are not integrated into the Sword World Confederation; they are subject worlds.  The Confederation heavily taxes Entrope to fund military actions elsewhere, but does not let any of the three Entropic worlds have militaries of their own.

This situation endures for about a quarter of a century, until the Fifth Frontier War, when the Darrian Confederation liberates Entrope.  The Sword World installed dictator is executed and Entrope chooses a new leader themselves.  The new leader is a war hero in the battles to evict the Sword Worlders.  She has been installed as a dictator also, but with the promise of eventually restoring the original technocracy.

As of the current date of 1120, Yoldat is successfully rebuilding Entrope's economy.  A sign of that success is the starport which has already been reclassified as type D.