Normally in the last quarter or the year. I think if you got the 6s now you'd kick yourself because it's around mid point between the last release and the new release, but that's my 2 cents...
Are you just wanting to port across the contacts, messages, etc?
I'm not sure about using the 6 backup on the 4. I know you can port contacts etc through iCloud though
So you're wanting to have your US account on 1 device, a SA account on another device, then use family sharing to "connect" the two? I don't see why that would be a problem to be honest.
With regards to seeing your fiance's apps, you wont' see them, I have family sharing with a few people set...