Sure they run out of fuel, the question is, will they do so before you run out of air
Also going through der Kanal in 39 is doable, and if your not in the narrowest shallowest bits, and are harassed by elcos, dive! otherwise I had a great time staying surfaced and dodging half-asleep DDs while staying decks awash. great adrenaline rush
concerning Tommy and Pierre coming for you, sure, but when they do you may already have made a significant dent in their merchant marine because those are also all in der Kanal for some reason. just don't waste any battery juice going after schooners doing 9 knots.