3. Can a tank blitz through an enemy’s neutral territory if the territory has no unts in it?
If the neutral country have been activated on a previous turn and there are no longer any enemy units present in it, you can blitz through it. At that time it functions as enemy land. The key here is activated and no enemy units remaining.
You can’t activate friendly neutrals this way, activation of them can only happen in non-combat and ends your movement.
You can’t blitz through a non-activated enemy neutral or true-neutral territory. Entering them in the combat phase is activating the units present there and therefore stops your movement.