If anyone here might know, just how badly can a steam locomotive's frame be bent, twisted, etc, and still be repaired? And in the case of a twisted frame, how would such damage be repaired?

As I recall, #1223's frame, when viewed from above zigzaged 7/16" compared to string lines from the cylinders.

Even that was within the amount that could be compensated for by custom machined shoes and wedges, so with careful work, it was no problem. In my career, #90 was the only engine I ever overhauled that still had a straight frame.