Suppose US controls Korea and sz6. Can Japan make a combat move into sz6 including transports which, when the sz is clear, will load units from Japan and unload them on Korea for an amphibious assault?
Nope. Transports are loaded during the Combat Move phase. Since you can’t load transports if the SZ is hostile it is impossible to move them to SZ6 and load them.
When you conduct a combat, there’s a sequence you follow. The decision to retreat comes after both sides fire, so you have to conduct at least one round of shots before you can retreat.
Yes, you can end up moving farther than you would normally be allowed to.