Amphibious assaults can be done in two steps, consisting of a sea battle and a land battle. In your example, both the cruiser and the transport would attack the destroyer. If the attackers win, the transport can offload and the land battle will occur. If the transport ends up retreating (or being sunk), the land battle will not occur and the transport may not offload.
1. German troops in Belarus are attacked by 3 infantry from Russia and a fighter from Karelia. After the first round of combat are the troops allowed to retreat to Karelia?
2. And same question if fighter came from Finland through Karelia?
3. Can you retreat into a just captured territory?
Only if that territory was captured by a blitzing tank that moved into the battle being retreated from. Blitzed territories change ownership immediately.
I mean if Ukraine is just conquered and attack in East Poland with some infantry from Poland and a fighter that attacked after flying over Ukraine. Are you then allowed to retreat to Ukraine?
As I read the rules on page 19 all these should be allowed, since the rules say that for Sea units the zone must have been friendly since the start of the turn.
The rules say this:
Move all attacking land and sea units in that combat on the battle board to a single adjacent friendly space from which at least one of the attacking units moved.
The “attacking units” in this case refers to the land or sea units that are being moved. It doesn’t include any air units, as they are not being moved at this point. Air unit movement does not provide a retreat route. This isn’t the best wording, so maybe an FAQ entry is called for.
Did Spring 1942 introduce defenseless transports?
Spring 1942 was basically A&A Revised played with AA50 rules. 1 or 2 Battleships and/or Destroyers were changed to the new Cruiser piece, but otherwise the set up was identical.
"Step 2: Roll Research Dice
For each research you have, roll one die.
Success: If you roll at least one “6,” you have successfully made a technological breakthrough. Discard all your researcher tokens and continue to step 3.
Failure: if you do not roll a “6,” your research has failed. Keep all your researcher tokens and continue to the Purchase Units phase of the turn."