1. SU partisans: If the German player splits the SU with Japan, than the SU as a nation cease to exist, so no partisans. Sad engine limitations, if you want to play beyond the SU surrender, than it is strongly advised to not split the SU, so the partisans stays, and you are forced to garrison, or agree with the Axis player with a number of garrisons that needs to be left inside the occupied SU.
2. Gibraltar pass: Again, engine limitations, sadly a pass hex can only be used by one side only. Yes, there could be a few more preplaced for the allies for this case, but that would be stange too.
3. US partisans: Noone knows the actual partisan numbers for the US in an invasion. Usually everyone thinks they would go out and stop an entire invading army, but currently we can see that the US population cannot stop even a few looters.
4. Italian help: Germany did not send any help to the italians, they took over some of the italian failures and finished as their show, but never sent money or equipment to the italian army or state in any meaningful number. Yes, it could be an option, preferably with some cost for triggering it first time.
5. Since there are zero reason to keep that unit on iceland, it your be just a cheap bomber used elsewhere.