All very true. So if the Landreich and Firekka Alliance are so affiliated with Confed, which powers make up the so-called Terran Alliance?
It's not clear exactly what forms the Terran Alliance; it's the equivalent of what you would have called the United Nations in 1944 -- a group of nations/races allied in the goal of defeating the Kilrathi. The term only shows up in Wing Commander Armada and in the Andropolos timeline.
Union of the Border Worlds - coalition of planets not aligned with the Confederation or under it's authority, nor a part of the Terran Alliance, however at peace with and with the potential to be allied with the Confederation.
The Union of Border Worlds and the 'Terran Alliance' wouldn't exist at the same time; the Union came into being in 2673--before that point, their worlds were part of the Terran Confederation.
Landreich Colonies - coalition of planets not aligned with the Confederation or under it's authority, nor a part of the Terran Alliance, however at peace with the Confederation, but unlikely to ever be allied with it in the future.
The proper name for the nation is the Free Republic of the Landreich; their independence was recognized in 2652, so it's certainly possible that the were part of the Alliance (they were both fighting the Kilrathi, they received Confederation surplus and of course ultimately assisted Confed at the Battle of Earth... and had the option of sending representatives to the 2668 armistice signing.
Firekka - coalition of planets? (I believe they had at least one colony? Someone correct me if I'm wrong) that was once a part of the Confederation (and thus the Alliance). Currently Independent with strong historical ties to the Confederation, potential Ally.
They're the "Firekkan Planetary Alliance"; the one colony system was settled after the war and after Wing Commander IV (T'kirsa, in 2675). They were a former member of the Confederation, but I don't think there are strong ties anymore - the feeling seems to be that the Confederation left them out to dry repeatedly.
Kilrathi Clans - surviving members of the Kilrathi race, at peace with the Confederation, never a part of the Alliance or the Confederation, future status unknown.
The government recognized by the Confederation is Chancellor Melek's "Kilrathi Assembly of Clans"; in reality, there's a violent five-party civil war being fought in the former Empire.
Grovsnor Colonies - I honestly have no idea what their affiliation is or was, only that they are separate from the Confederation. I believe you do aide them in the course of the WC games, so they are possibly a member of the alliance.
"Grovsnor Colonies" is simply a one-off reference in Fleet Action to another group that is in a similar position to the Landreich (someone notes that the Landreich and the Grovsnor Colonies didn't send representatives to the armistice signing).
I wonder, where does the Pilgrims and Blair's Pilgrim heritage fit into this Border Worlds issue?
It doesn't; Nephele was not a member of the Pilgrim Alliance. Rather, it was the home (at the time of his birth) of his father, Arnold. His Pilgrim heritage (genetic or otherwise) comes from his mother, Devi Soulsong, who was from Peron (Luyten). In general, the former 'Pilgrim' worlds are much closer to the core than systems that became the Border Worlds.
(Now, behind the screens, the first draft of the movie concept had 'Border Worlders' as the group with the special abilities instead of the Pilgrims.)
WC1 and WC3 give different origins for Maniac's origin too, WC1 claims he is from a colony, and WC3 says he is from earth, but does not go into detail. But maniac is fully human(as opposed to payback and several other unnamed pilots from WCA), so he would be referred to as an "earthling" anyway.
It has never really been clear what Payback and company are -- some alien race, humans who grew up under some kind of different star or what.
My take would be that the 'homeworld' fields in Wing Commander III and IV (in Victory Streak and the wingman selection, respectively) refer to the planet the character currently lives on, not where they were born (the big giveaway is Flint, who Victory Streak lists as Earth instead of Locanda; I could believe Chris McCubbin would forget Maniac's story about 'Leto' from Claw Marks, but not that he'd forget how important Flint's birth world is to WC3).
Blair was born on Nephele and moved to Peron at a very young age (he was there for the siege that killed his parents). He is adopted by his aunt and goes back to Nephele for most of his childhood. As an adult, though, he had a home on Earth - he uses a trip home there as a cover for his disappearance in one of the Special Operations stories and then he returns there at the end of Wing Commander III... after that, he moves back to Nephele to hide (before WCIV).
No question that Maniac was born on Leto, in the Proxima system... but he seems to make his home on Earth (in WC3) and then Mars (in WC4) as an adult.