So a few thougths...
"putting up snorkel or 'snort' masts (to run diesels) is suicide as airborne radar can sweep huge stretches of ocean"
Right, so how about creating a bunch of mini-torpedos which you set to quietly motor off in various directions and, after a set amount of time, they stick up a mast with a radar reflector on the top and suddenly you've got snorkels everywhere! (Oh, and add in a sound generator to confuse the Sonarman Jones's of this world ;-) )
As for "Responding manned units starting from the "flaming datum" of the destroyed ACTUV would soon find and destroy the sub as it crept away - all the sooner if it surfaced or put up masts in order to flee more quickly."
Right, another mini-torpedo, but this one's job is, for instance, to latch onto the ACTUV with a magnetic grapple and then steer it off in another direction or just apply reverse thrust to keep it in place for a while then, after a set period of time, explode.
For every "cunning plan" that someone comes up with, there's an equally cunning counter :-)