With the fluid being over full while the car was running, after you lost movement, that tells me you lost pressure and fluid was no longer being pushed through the transmission. Whether it be from the front pump, the converter, or an internal part in the transmission failed causing no movement. Something quit functioning properly.
You’re to the point where I would completely disassemble the transmission, inspect every component with a fine tooth comb, and if all checks out, re assemble. If everything checks out, I would get the converter swapped out, even if you have a loaner, just to see if that fixes the issue.
If the stator in the converter fails, which is the fluid coupling device in the converter that multiplies the torque, I believe you would lose movement. Especially after doing a stall speed test, which is hard on stators.
I am to the point of spit balling because if someone came to me with this problem and the transmission checked out, I would be looking at the converter as the failure point.
Kim, thoughts?