Slant Six stalling when foot on brake & engine warm - transmission issue?

I sortof agree with your diagnostics
Here is what you need to know;
Reverse is created by the application of the Hi-drum and the L/R band.
All forward gears go thru the forward clutch, which Chrysler calls the "rear" clutch.
After that,
Third gear adds the Hi-drum, while
Drive-Second disengages the Hi-drum, and engages the "K/D band, and
Drive-low, goes thru the rear sprag while Manual low re-engages the L/R band.
With the factory VB, you cannot get past First gear until the Governor pressure comes up high enough, which makes diagnostics for your case, difficult.
But I'll give it a try.

If the engine gets dragged down in reverse,
Either the convertor is bad, OR, the Forward clutch is.
If the engine gets dragged down in manual low;
Either the convertor is bad, OR, another clutch pack is engaged, probable it has been locked up by cooking it.
At this point, your only choices are the Hi-drum and the K/D band.
If you could get it up to speed, and you found ONE of those two gears worked perfectly, that would be the locked up one.

At this point it doesn't really matter cuz the trans is PROBABLY gonna have to come down, and inspection should tell the story. BUT,
You could perform an air-test , which involves pressurizing all the servos and bands one atta time and seeing/hearing what happens.
If you can prove that they are all working and the oil does not stink, well then, the TC sorta must be the problem.

If you get to thinking that the TC is the problem, you can get the rear wheels off the ground, start her up and run the speed up, if it now shifts every gear pretty much normally, that points to a failed Convertor. Then, as you brake the wheels to a stop, and that action again brings the rpm to near stall, that proves it.