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Those aren't bad guidelines, but there probably wouldn't be much of a practical difference if they were implemented, or it could backfire. On the first point alone, if a shitposter got banned for plagiarism, he wouldn't get much support from other members or he could employ a team of alt accounts or "friends" to rush to his aid.
I think it's probably best to just accept that Theymos is going to be the arbiter of ban-lifting. It's a centralized solution, but he's pretty level-headed and knows what he's doing. And he's the admin, too.
I believe if collected merit of voters would be considered - that it would solve the problems of alt account voting. The users with more collected merit would have more "voting power". I doubt that alt accounts have much merits.
It is up to moderators to decide if the banned person was "net positive for the forum", and I believe it is hard task in some cases to decide if someone is good contributor or not, but proposed criteria would certainly make their jobs easier. Also it would allow that voice of community is heard, specially of communities in local boards that are not involved in forum politics.