i will try to answer some of what i understood.

Steem will be still here, and no one knows what will happen. we can all just speculate. i think i will start powerdown, but what then, not sure.

snapshot means that they will be copying the chain at that exact moment, so you don't need to do anything. they will do it for you.

Steempeak said there will be a front end for hive. splinterlands said they will wait for the chain to stabilize but if all is ok they will migrate. heard a lot of daps that will go to hive.

keys will be the same as keys on steem. you could change them, but i think (and this is a noob telling you :D ) that there is no need for it. so you just log with your steem keys on hive frontend (herd that there will be a keychain for hive)

never thought about rep. but as it is a copy of the chain, so most probably it will all be there.

if i can help with something ask, not that i will know but i will try :D