You should have a look at the sticky about yeast and cider experiments.
It's pretty amazing, really, the amount of info there.
99% of what I know about cider, I got from there.
Back sweetening: Do it of your cider is too bitter/acidic, or you like sweet cider
You don't need to kill the yeast for kegging. I cold-crash mine, and keep it cold. I use ale yeast, so anything leftover is not too perky in kegerator.