Not sure if you are expecting answers, nor do I know much about all these, still let me spill out somethings:

(1) Mastodon seems chatting while Pump.io seems, eh, blogging (?). So it depends on which mode one dislikes more.
(2) I don't know. But I guess the paint point will be keeping intact and functional the follower/following thing. Also Pump.io allows richer markup, so converting from legacy posts to just text (with some links and pics) could be headache.
(3) An art project indeed. One need to have both personality: you don't feel like talking and you are afraid to write. I think an empty space will do.