My interpretation of the rules regarding this situation:
In the euro 40 rule book it says that when you build in a hostile sea zone, combat does NOT occur right away because the conduct combat phase is over. However, I do not know when that combat should occur. I thought (hoped really) that the combat would happen during the Italians turn, preferably with them defending. This is why I bombed the airbase, if they were defending there may bot be a scramble.
To answer your questions:
If you get to attack me, the Italians would not participate since there are no multinational attacks. I would think that because of that, you do NOT get the benefit of the destroyer, and therefore could not use planes (is my bias showing?). If you were defending that the Italians would help you of course.
Your transport unload question: (copied from http://www.harrisgamedesign.com/phpBB3/viewtopic.php?f=40&t=6149)
“transports are not allowed to unload land units for an amphibious assault in a sea zone containing an enemy sub(s) belonging to a power with which they are at war unless at least one of his warships was also present in the sea zone at the end of the Combat Move phase.”
So you can’t unload without YOUR warships in that sea zone
You attacking me and the Italians not helping seems kinda silly to me, but I think that is how it would work. I am tempted to make a post in the G40 FAQ thread on this actually, because something doesn’t seem right, and I think the combat should have already happened.
In short:
I think Germany cannot unload without German warships, and you need a German destroyer to attack with German Planes.