THINK we're getting an offer on our house tonight

Sorry, dint see post about shower on wall on backside. I have never seen showers back to back on same wall as long as I have been into constr.

Dd, is this a fibre glass tub, or shower unit. If it is then its gonna be extensive work, take longer than a day if you have drywall around a fiber glass unit. Post me a pic of the unit man

DD, do you think it's possible iron build up?? Don't know how old your home is, but a lot of times in older homes ive worked on in the past, fixtures that aren't used often iron will collect and you can't even turn a knob.
If this happens to be your case, start by flushing it with vinegar, second option is acid (muratic). It'll change the form of the iron from solid (disolve) long enough to flush out the fixtures, valves, etc.
it's a pain, but when your working on antiques that are priceless to the owner.. That's what you gotta do. In your case, it would be easier to flush it out, if its a build up, than tear into a "back to back" shower project to replace.

Took the diverter out and they don't even make that diverter any more. I went to several plumbing stores, ace hardware, lowes, speciality stores and it's an odd one. It's a fiberglass shower unit and I did have a guy look at it while he was there doing something else and he said it had corroded really bad on the inside so the unit would have to come out and have replace the parts behind the unit or something like that. I capped it off, took off the shower head, capped it and installed chrome caps on the fiberglass unit to make it look purdy. Just a tub but doesn't leak and won't leak. It may or may not be a problem for whoever is buying the house. not sure. HOPE NOT though but if they need to take a shower, they can use the other bathroom. Me, I take a bath, grew up that way and always have taken baths

Mechanical objects with moving parts require proper maintenance.

Kinda hard to maintain when they're in the wall.