@General:
4. Can Germany attack the UK ships in SZ 106 on G1, without provoking the US into war? Especially since there is a convoy in that sea zone.
There is no way you can draw the USA into the war by disrupting UK convoys. Forget about the fact that USA is shipping stuff to UK - that is irrelevant for gameplay purposes.
5. Can you disrupt convoys without being at war with that particular nation? Can f. ex. Germany disrupt the convoy in SZ 106? Or the SU disrupt the one in SZ 125?
Never. You absolutely must be at war with the target nation to disrupt their convoy. When you disrupt 106, that is a UK convoy only. Convoys go with who owns the adjoining territory - don’t even think about the “other end” of the convoy - it is irrelevant.
6. Do you have to make a DOW at the beginning of your turn to be able to disrupt a nations convoys in the non-combat phase?
As Jenn said, if they declared war on you previously, then you are automatically at war with them anyway. So the answer to your question is “yes, unless they declared on you first”. Example: Germany declares war on Russia on G2, and Russia had moved a sub to Z125 on R1. If Germany does not destroy that sub on G2, Germany’s income will be reduced by 2 even though Russia has not declared war yet because she hasn’t had a turn yet.
Per page 9 of the Europe manual, you are supposed to declare war at the beginning of the Combat Move phase. So if you are the one initiating a state of war, you would declare war in your combat move phase. Convoys are disrupted during the TARGET’s turn, not the aggressor’s. So if at the end of the target nation’s turn the disrupting ships have not been sunk, they will receive less income as a result of convoy attacks.
You could move your ships to the target zone (with the convoy symbol) during your non-combat move (or combat move) but that is not when the convoy is disrupted. It’s not until the collect income phase of the target nation that the convoy is effectively disrupted.
Jenn adequately answered 1-3 and I am in agreement with her answers.
Krieg is away and has asked me to watch the FAQ for him. When he comes back he’ll read all questions and answers and make any edits or additions necessary.