if its not one thing, its two! thanks for the details on troubleshooting and that cure all OVP is amazing. Now on the trans I had a 95 E320 which had been sitting and lost reverse gear. Adding fluid helped, then change outs with castrol hi mileage fluid and trans X additive brought back reverse, then it vanished again and finally I discovered Blue Devil sealer which is a seal revival additive labeled as stop leak. Again reverse gear came back from the dead.

On one occasion I overfilled the trans and reverse was strong but I lost the forward gears and got flaring. Then it slipped in forward gears badly and overheated. I reduced the fluid levels and all was well.

Also had issues with the vacuum modulator valve..

I cant say on the trans failing to shift but just add the info for what its worth and also comment that I swear by pentosin fluid which I changed out to for the end use..Im not trans expert but I know how sensitive to fluid condition and levels they are.

Best of luck! Keep posting what you find.