And does anybody have the procedure for flushing a torque converter? I really want to know if this is common knowledge.

Did you ask the person who sold you the transmission how to flush the torque converter?

Honestly, he was really hard to get in touch with. I got the impression it was a guy fixing the transmissions on his own.

I wonder if that might be the problem...

Hi, I know you have already said that you don't think it is the parking brake, but I had a situation on a car where one of the brake shoes had come away from the backing plate. The car wheels would roll OK (ish) when rotated forward, but would bind badly in reverse. It might just be an idea to take of the brake rotors and confirm that everything is OK in there, as it is easy to do.

I actually have seen the car and now the party involved.
I do not think the brakes are the issue. He has jacked the rear up and the rear moves freely.
He has adjusted the shift linkage. When I was there it was off. No neutral at the time so he followed the procedures to adjust. All the gears are account for at this time.
At the time and the symptoms I thought it might be the throttle linkage ( no power seemed go to the transaxle which seemed to be something "this" might be a cause). According to the party he followed the procedures to adjust. He is confident he accomplished this task.
I had him check the flex dampner ( i believe in the bellhousing ) to check if that "might" issue. When I heard car running no sounds of flex damper going bad was evident but just to be on safe side he checked. No rubber particles or evidence found of dampner going.
When I eye balled the transaxle I was suspect as it was so dirty for a rebuild. We have talked and I do wonder if this guy might have "not" done something and he received a bad tranny. I am not sure. I believe he is trying to contact person.
I am not sure what is left to do.
I have talked to a couple of Porsche Mechanics and all we have done has been suggested short of finding another transaxle and see what happens.
The car ran just after he install tranny and then nothing ( how with the shift linkage out of wack I am not sure) Something went out in 15 mins. I still wonder about the throttle issue.