Eh but 2 does not state it has to be a combat move. And since submarines threat hostile seazones as friendly the zone is effectively not a hostile zone and number 3 allows the movement. I would say Yes to question 3 based on this. Moving through a friendly seazone is not considered a combat move and the SZ not containing destroyers is effectively a friendly seazone for a sub.
Well, I’d be willing to advocate both views. As I quoted submarines treat hostile sea zones as friendly, but that statement is under “combat movement phase”. Looking under “ncm phase” this is not repeated, in fact, it states that NO sea units may move through hostile sea zones EXCEPT in the case of enemy submerged submarines.
Might be worth looking more into, but I assume mostly for theoretical interest.