1) Yes, that one domain can hold multiple "clients". I have 3 different forums running through my sendy installation.
2) ^
3) Setup an Account via AWS. Not to hard.
4) No needed in my opinion.
5) I use my own add on, but you can build the newsletter in Sendy, and the "list" in Sendy is sync'd...
I'm maintaining this to work with each XF version in that regard. As far as features, I know of the one you've requested I've just not got to it yet. Happy to listen to more if/when I get time.
If I remember, that one was a lot more feature rich, but use to cost money. Mine was a "lite" version but free.
If that is open source now, it's probably a good option and this add-on is not needed.