1) Yes. Subs fire simultaniously, at least according to what moderator Switch has told me.
2) Fighter has 2 movement spaces left. It launched, moved two spaces, then attacked.
3) Fighter gets 1 space to land in friendly territory.
4) You may retreat to any territory you attacked from, and all units must retreat at the same time (except is air power can retreat before land units). So yes, the tank can retreat whereever and the infantry whereever as long as at least 1 unit originated from that territory.
I use that all the time to get 2 movement with my infantry. I attack, say, Ukraine from Caucasus and E. Europe, don’t kill everyone and retreat to E. Europe thus moving 2 spaces instead of 1 with all my infantry.