It sounds like you've covered all your bases as far as electrical connections go, but try checking the voltage at the positive cable at the starter to make sure you're getting full battery voltage there.

Also, I don't recall if you've said that you've done this yet, but have you tried cranking it while a running vehicle is connected via jumper cables? That would ensure that you're providing full cranking voltage the entire time you're cranking it.

On the subject of bellhousing not being square, maybe the PO removed some of the bellhousing bolts for some reason and the starter is binding on the flexplate. Have you looked at the transmission to engine connection to make sure there's no gap and that all the bolts are there?