Update 4/17/18:
Having failed to get him by phone this morning, I decided to try via text.
I informed him that I had decided to crate his transmission up as it had come and to return it; I further explained that I wanted to simply get a refund.
I figured he'd not be pleased about that, but he instead replied that this was acceptable to him.
I then asked that I get a "will call" prepaid shipping label, which he also readily agreed to.
I had already contacted the mechanic earlier in the morning and asked if he wanted to give my old transmission a whirl at fixing, which he readily agreed to and stated he would bring his own "expert" in on the process as well - but there would be no charge or estimate until they got mine apart and diagnosed.
Works for me. 
Somewhat surprised, I told Jamie what the plan was with my old transmission and that I'd just like to go that route, rather than risk the shipment of yet another unit from him.
I mentioned my out of pocket losses of yesterday for towing and the mechanics' time; I told him the new plan was that once my old transmission had been repaired, we would tow the GTX back to the mechanic, where we'd remove the Passon unit from the car and put it back in the crate, then install my old one in the car, hopefully ending up with my car back as whole and the Passon unit ready for shipping.
He replied that he was fine with a simple return and that I'd be receiving a FULL refund of everything I had paid him; he asked that I let him know when the
unit was ready to ship, when he would email me the shipping stuff to get ready for UPS to come get it.
I was quite relieved to read that message, received while I sat in the waiting room of the endocrinology department of the major hospital in the area - as you might have surmised, the next episode of "Ed's body is failing" is currently in progress, but I digress....
I replied something like "atta boy, Jamie, there ya go" and he said that this had been the plan all along.
I replied that I understood that stuff happens and that I know he's busy as hell, but that the previous days' interaction had left me non-plussed to say the least.
I offered that he might want to get some help and he replied that he'd work on that and was sorry things didn't work out.
I am too, honestly.
Bottom line as of now:
1. My old transmission is already in the hands of the mechanic, who will contact me when they figure out what's up with it.
2. Jamie Passon stood up and is doing the right thing here.
What a difference a day makes, eh? Yes, I'm very relieved and content for the moment, especially in light of the news I got at the docs'.
It's always good to get a little bit of ones' faith in humanity restored and Jamie did so for me today.