I’ve just noticed that the rules apparently allow NCMing a sub into a SZ defended by a fleet which includes a hostile destroyer, unless you argue that the rule which states that DDs nullify the “Treat hostile SZs as friendly” ability overrules the rule in the sub unit description which states:
Treat Hostile Sea Zones as Friendly: A submarine can move through a sea zone that contains enemy units, either in combat or
noncombat movement. However, if a submarine enters a sea zone containing an enemy destroyer, it must end its movement
there. If it ends its combat move in a hostile sea zone, combat will occur.
This appears to imply that you can indeed move a sub into a hostile SZ on NCM even if it contains a DD, you just can’t move through it.
Triple A allows this move FWIW.
This strengthens this play, a fair bit IMO because you can’t just have a half the number of fleets as SZs to protect.