So... here is what the shop said today about my car...

We have been working on the engine today and have determined that the likely
cause of the leakage is twofold:
 
"1. The rear main seal wasn't quite straight which caused oil to leak out
into the bell housing - probably your "big" leak.
 
2. There appears to be a problem with the casting of the engine case which
may cause it to leak some when quite hot and under load. This is basically a
small, thin crack that only opens up under high heat. It is so small that it
took quite a bit of effort to spot it. I don't think it is terribly
detrimental. Scott beleives it can simply be patched. This would probably
work just fine and is an inexpensive fix. The "correct" remedy would be to
replace the case (reusing the internals.)
 
We are going to look at costs, discuss the situation further this afternoon
and then present you with some choices."

So I am waiting to see what the choices are... the crack in the case was really the only thing that's made sense to me this whole time and was also the thing i feared the most...

No idea when I got a rear main leak as I was only leaking oil in to the engine compartment so far as I knew...

can you patch a crack and have it actually last?

I rest my case( no pun intended). I haven't seen a case stress crack that was welded that would last for any time. I would love to know where they found the crack. Make them tell you.

Those stress cracks are a "booger" to find sometimes.  We use to strip the tin off the engine, re-install the intake, bolt a tranny case to the engine and fire that dude up with it sitting on the floor. With no cooling it doesn't take long for the crack to show up maybe 10-15 min.
    Glad things are working out after all the crap you had to deal with Pancho

Like I said before, If this crack is on the oil galley side of the case around the cooler area, take a hammer to it. There are to many good cases around to take the chanch of patching it and having it fail for you or someone else.