1) Sure. Inactive members probably wouldn't mind being removed, just make sure if they decide to come back that they're able to be "re-instated" immediately.
2) Yes, they should be removed, but not without definitive proof. Anybody who isn't playing by the rules just to climb the ranks of a leaderboard is kind of pathetic. I don't care if they have a separate gamertag which they use for pirated games, that's their own prerogative, but don't use it on a leaderboard.
3) Great idea for two reasons. It makes it easier on you, and meta tags make grouping/meeting for games possible among many people with only 1 of the 100 friend slots rather than 10-20 or whatever. I have a few of the members already added, but if they were friended by a meta tag I could just use the "View Friends" option to get access to them rather than keep them on my friends list individually.
4) Merits other than post count should be considered; if they have decent scoring flash, if they submit art/audio, etc. ONLY going by post count encourages spamming just to be allowed in.
5) Sure, I wouldn't mind seeing this thread being pruned; if the first post was "set in stone" and absolutely correct it should help prevent any confusion of potential new members who were interested in joining. There's no downside to it as I see it, anyhow.