This is complicated stuff SC, but I believe I have this right:
In your example, if the US initiates combat in a sz in which the US has no destroyers, then J decides whether its subs submerge or surprise strike, not the US.
If the US had a destroyer present, then any subs in a sz in which combat is initiated would be involved. The US cannot decide to ignore them.
If the US placed units in a sz which contained only J subs, then the US can decide whether to initiate combat or ignore those subs. If that US movement occurs during non-combat then combat cannot be initiated.