1. Yes, but both sides will have the option to submerge their sub before any shots are fired (as neither side has a destroyer to prevent the other from submerging), so it’s unlikely that the defender will stay to fight the lopsided battle.
2. Yes, subs from opposing sides can share a SZ.
3. Yes. Russia doesn’t get the NO if there is at least one German, Italian or Japanese sub in SZ125, even if they are not at war with the power owning the sub.
4. No. You need a surface warship. They are destroyers, cruisers, battleships, and aircraft carriers, so no subs or transports count.
5. Yes. UK shares all tech between the two economies. Also, both sides can contribute when buying tech dice. For example, UK Europe can spend 4 IPCs and UK Pacific can spend 1 IPC to purchase 1 tech die.
6. Yes. If AAA are the only unit(s) remaining on the defender’s side and the attacker still has at least one unit, they are automatically killed and the attacker conquers the territory, if applicable. Note that the attacker does not get a chance to retreat from only AAA, so you can potentially strand attacking ground units that were intending to retreat by taking all non-AAA as casualties and leaving only AAA to be auto-destroyed.
Hope this helps!