my overall reaction to your post above is a hearty "meh". i wont dismiss your points, but i will dismiss the need that they be answered in a fashion acceptable to you. much of what you're looking to be resolved is unnecessary and somewhat irrelevant, so any response would just be a detail war.
as another fan of the shield, i could very similarly attack the finale with a barrage of detailed questions about any number of aspects*, and i'd hold myself to the same "how does that even matter?" response. (*why did nothing ever come of the cat-killer dutch, why was acevata not more heavily implicated in any of vic's reveals, is vic willing to let ronnie die in prison beatdowns despite going all out to save other friends, etc. ...and no, i don't want you to answer these.). the point is, you're not supposed to be looking for tie-ups to anything, and there's not a single series or film that does that. just try naming one, and we can shoot it to shit as a family.
generally speaking, my interpretation of the show's purgatory was simply a vehicle. a matrix. a timeless world to reconnect, introducing even seemingly odd or unflattering characteristics to help jog their own memories (obsessions, criminal records, etc.). maybe this included a bit of wish fulfillment; sawyer going from con-man to dharma security to cop, jack going from loner to drunken adoptee to a dad.
the qualifications for being there could be anything. maybe everyone there, even those they interacted with (alex, rousseau, anna lucia) were also dead. maybe they were just figments of the losties' dead imagination that disappear when the island crew does. maybe everyone gets to have this special chance. maybe it's just the island people.
to me, those are interesting questions, but not even remotely necessary ones. all we know (and my argument; all we need to know) is it was the island people that had the special connection, and they were the ones featured in the church.
these are not good enough answers for you, nor would my answers be to your other questions (and i'm happy to share). but i think the fact that they
are answers, is key -- they did address what you wanted them to address, you just didn't like what they came back with. and that should shift your argument from "they fucked up and didn't address anything" to "i hate why they did this"
