Can Soviet invade Finland on their first turn, without Germany declaring war to Soviet first?
No. USSR may not move into territories outside its own on the Europe map until it is at war with Germany and/or Italy. Also, it may not move into territories outside its own on the Pacific map until it is at war with Japan.
Regarding flights/landings, are islands still part of the sea zone? Meaning, does it take an aircraft 1 turn to enter the seaszone surrounding the island, or does it get that “for free”?
Islands are separate spaces from the sea zones that they’re in. It takes a movement point to enter or leave an island.