727 - Let me describe my symptoms...ugh

Lots of good answers.
If the shift quadrant is properly indexed so that the manual valve is properly detented, then
I think it's past oil pressure testing, and for sure the pan should come down.
Locked in reverse has only one cause, the band is applied. Could be mechanical failure. Could be hydraulic. At least the high-drum is working,lol.
But I'm confused about the excessive overlap on the 2-3 shift, which you say came first, and has nothing to do with the reverse situation. So... an air pressure test might help,
but as for myself; I'd be taking it down for a full tear-down. In about an hour to 90 minutes, it will be on the bench with it's guts spewn all over the place, and a full diagnoses made.