Seriously at my wits end

I had a similar problem with my 727 a few years ago.

Turned out that The o-ring on the low/reverse piston pivot pin was broken and leaking.

The pin is inserted into the transmission case while The tail shaft is removed.

Requires a lot of tear down to replace it.

You may be able to seal it with some silicone from the outside as a temporary fix.

From underneath the car, look up just above the pan rail. It's the rail that is closer to the rear of the car. To the right of center (closer to the passenger side) you will see the end of the piston pivot pin. It's about a 1/2" diameter pin, but partiall blocked by the flange on the tail shaft. The tailshaft is what is preventing the pin from sliding out.
Clean the area and apply some silicone to see if that stops the leak.

During a rebuild, you would slide the pin out and replace the o-ring.