The word from the transmission shop today is that my old unit (the one with the mystery noise, remember?) is only in need of two (2) main bearings (I guess that would be "both" main bearings, right?) and a front seal.
That's it.
As I guessed, all the gears and synchros were in real nice shape, lending to why the unit drove so nicely.

Those who guessed "rear main bearing" or some such, come forward and collect your prize!

I'm no transmission guy; they never interested me much, honestly. I much prefer any/all other parts of these cars to repair/rebuild and have done so many a time.
Even so, this answer does satisfy me that the usual logical progression of eliminating what a problem ISN'T eventually leads to what it IS.
It did so in this case, certainly.

We'll swap out the transmissions Monday and get the Passon unit on its' way back to Jamie and hopefully the car behaves itself after that.
Fingers crossed.
I need for it to work; really need a break here.