Transmission Issue related to P0713

[X5 THANKS DAD]

Ok, folks, short term lurker, finally became a member today.
I have a heavy automotive hobby and the facilities at my residence to do pretty much any mechanical or electrical repair, plus limited body and glass repair. I don't paint as a spray gun is just a weapon in my hand.

The preliminary stuff:
I bought a 2005 X5 3.0 Auto transmission down in Maryland that ran fine, but did absolutely nothing when placed in any position on the shifter. No movement, no noises. Vehicle has a really nice body for 167,000 miles. Instead of buying a used tranny for it, I picked up a rollover in Cleveland with identical model year and powertrain; 120,000 miles. It lot "moved" when put in gear, but could not really drive it due to broken control arm and strut at LF wheel. It only had the 4x4 rollover mitigation light illuminated, which my have been from an extended period with a dead or disconnected battery. It did not have a MIL light on or "Trans Failsafe Prog" warning.
Rather than just swap transmissions, I dropped out the cradle and swapped the complete lower mileage powertrain and put the rollover engine/trans/transfer case in the good body, with the exception of the front suspension. I actually split the oil pan off the bottom of the engines and mated the original front suspension/knuckles/rack to the new power train, avoiding disassembling the axles and front final drive through the oil pan. (That was fun while the engine and powertrain were hanging on my shop crane!!) I did bring the rollover transmission control module along and used it in the new body, keeping matched TCM and transmission together. Put it all back together, and what do you know, it fires right up.

The issue:
I had no codes until the engine temp gauge stabilized mid-sweep, and I assume its at operating temperature.. At that point, I got a "trans failsafe warning" followed by the 4x4 rollover mitigation light and then an illuminated MIL. The transmission had the forced downshifts as I slowed down to a stop.
Came home and pulled the code P0713 "Trans fluid temp sensor A circuit high input"

Did some searching here on the forum and found that replacing the harness inside the transmission case, which apparently carries the fluid temp sensor, might take care of that. Bought the new harness. Dropped my fluid, put in my new harness into each solenoid and changed the filter while I was in there (trans pan was clean with only a slight bit of material stuck to the magnet). Replaced the fluid. (Data scans during this time always showed a -40 degree fluid temp, like it is not seeing any signal.)

Took it out on the road again and when it reached operating temperature it again triggered all three warnings, including another P0713 MIL. I then changed out the complete external transmission wiring harness from the transmission case up to the oxygen sensors and over to the cowl bulkhead disconnects. Got in and drove about 55 miles/90 minutes with no warnings. I was happy as a clam thinking I had the issue nailed, when all of a sudden I start to slow at a stop light and feel the hard downshifts as I came to a stop, with the "trans failsafe prog" warning, the 4x4 rollover mitigation light on, and an illuminated MIL. Scanned it again and got the same P0713 code. Before that, it shifted very nicely, and the vehicle runs well otherwise. It has not thrown any other codes since I put it back together. I did also swap the temperature sensor in the cowl bulkhead disconnect "box" that is integral with the external transmission harness.

Anyone have any thoughts about where I might look next?

(Forgive me if I have yet learned all the BMW acronyms for certain components--I'll get there. )
I appreciate any insights anyone might be able to share.