Don't beat yourself up too hard. It happens. And it's particularly difficult to see down there to positively ID a leak. The pressure tester and a camera were keys to finding my leak. I recently spent a weekend tearing apart our Monte's engine to fix a leaky intake manifold gasket. In the process, I caused a new leak, misidentified the source (leaked on one side but ran along the engine and dripped down on the other side), and wasted more time and $ on the fix. Live and learn.

BTW... it was a pain finding a replacement drain plug. Couldn't just buy one from any local auto parts store for some reason. If your leak does turn out to be the drain plug, and if you can't find a replacement, post back here or send me a PM. I have two spares now and would be willing to part with one.