1, From personal experience it takes just short of one hour, to be precise 57min for 15 GB instance goign from small to large. Which I did not expect to be that long to be honest.
update: just learned that switching point in time backup before upgrade speeds up the process significantly
2, I would say creating MULTI AZ before doing upgrade would do the trick, hopefully that does not have downtime as well. Question is do they allow upgrading one without other...
3, yes, but I'm not 100% sure though