a) While they needed the snorkel if they were to run at any speed submerged (as it enabled the use of the main, diesel propulsion plant), sneaking up on the enemy was often done on batteries (slow; limited duration; no trace on the surface) - but if you were following (as opposed to lurking in front of) a surface ship, you'd definitely have to run on diesels.
The downside to this, of course, being that you'd announce your presence to any competent lookout on the ship you were following.
b) If I was German, I'd definitely prefer to surrender to a western power; I do think it may have been a bit rash to announce my presence in such a spectacular fashion with all the risk that entails, when I could as well have slipped a couple of miles away, surfaced, draped some table linen from the conning tower and radioed my intent to surrender and request instructions; much better than getting rammed or blasted out of the water for my trouble.
c) Three out of four U-boat crewmen got killed during the war, so it could be argued that being on a U-boat had its own risks - besides, this late in the war, the tankers would hardly have had any fuel to ship anyway - the Germans lost their oilfields (primarily in Ploesti, Romania and Maikop, Russia) prior to the endgame and largely relied on synthetic fuels, available in limited quantities.
That being said, I agree with your assessment that the U-boat force had ceased to be a serious threat way before the spring of 1945 - fuel shortages definitely being a major factor.