What are the odds that the seat stays are a different length or at least welded at the wrong place causing a length differential?

If it was a mixte: What if it had a seized seatpost at one time and whoever freed it, did so by twisting the seat tube there by pulling one seat-stay up/forward and the other down/back? Not that I've ever encountered this. But, but after freeing up a few seat posts by twisting the bike about in a vice, I can imagine on a mild steel mixte it could theoretically be possible.

It's pretty mild steel & there is no guarantee it was built to high quality specs/standards in the first place. I'd determine whether or not the seat tube & head tube are in alignment before continuing. That'll give you reasonable reference points to determine if/how the rear triangle is out of spec.