Good writeup, @anth@a.9srv.net! I agree to most of your points.
3.2 Timestamps: I feel no need to mandate UTC. Timezones are fine with me. But I could also live with this new restriction. I fail to see, though, how this change would make things any easier compared to the original format.
3.4 Multi-Line Twts: What exactly do you think are bad things with multi-lines?
4.1 Hash Generation: I do like the idea with with a new uuid metadata field! Any thoughts on two feeds selecting the same UUID for whatever reason? Well, the same could happen today with url.
5.1 Reply to last & 5.2 More work to backtrack: I do not understand anything you’re saying. Can you rephrase that?
8.1 Metadata should be collected up front: I generally agree, but if the uuid metadata field were a feed URL and no real UUID, there should be probably an exception to change the feed URL mid-file after relocation.