Hi itiosso, sorry to hear your problem and it sounds like a cracked head. It sucks that the PPI didn't catch the intermix condition, which btw should be quite easy to catch.

I would try to negotiate with them and see if you can get any help from them. Worst case if you need to repair everything on your own dollar, you will at least have to diagnose which bank is the problem (usually the crack can be seen from within the spark plug hole on cyl #1 if memory serves. The head can be repaired by Hoffman auto machine (Len is the owner and you can tell him I sent you). Whatever you do, you need to drain out the coolant asap as the oil will eat up the rubber coolant hoses and replacing them all (29 of them) will be very expensive.

It's best if you can start a thread to get more opinions and have more eyes on your problem.