 Was the belt just so worn and loose that the marked position was way off the expected mark? Sure. No way to be sure, as you point out. Had the previous mechanic, how could the previous mechanic have done anything wrong, that installed the leaking water pump and left the timing cover off, installed the belt a tooth off? Could the engine have been running okay that way? Impossible? Man, maybe not. There are some motors that will run, but they won't damage anything. They just don't run great. And then panic set in, and I couldn't know what was right.