Always On reaction in specific scenario
The steps you have should almost work. Assuming that there are database changes in the upgrade (I'm assuming there are because you want to keep SQL #2 offline so it doesn't upgrade) then SQL #2 won't be able to automatically or gracefully become the primary, as there is no quorum it can use. You'll need to force a failover with data loss.

For your questions: 


I can't see why it wouldn't work. SQL #2 doesn't know about the changes on SQL #1, the sharepoint farm doesn't know about the changes, everything should resume as normal
If you power on SQL #1, it will realise that it's not the primary, but it will also be impossible to make it synchronise again. You'll have to drop the database from #1, break the AlwaysOn Availability Group and reseed it again. Because #1 isn't the primary I can't see how it will write data over #2.