Sorry you are having problems. Note that if you had been bypassed, you wouldn't be facing the transmission problems you are having now.
I do not agree with the Nissan Techs at all. I work for a heavy vehicle manufacturer, and am involved in many technical aspects of cooling package design, testing and sourcing.
Although your statement is correct in regards to the efficiency of water/oil vs air/oil, you also have to think of the deltas:
Water = 180F to 200F vs Oil ~ 250F
OR
Air = -40F to 120F vs Oil ~ 250F
The temperature delta of Air/Oil is greater than that of the Water/Oil. There may be something to the 'heating up' effect you mention, but I'd put forward that when its -40, your rad is stone cold too. I think the transmission would heat up a whole lot faster than the radiator when driving. Maybe the transmission helps heat up the engine?
We have also had several members install transmission temp gauges to monitor the different conditions between standard setup, bypassed radiator, and bypassed radiator with external fan. In most operating conditions, the bypass did not seem to really affect trans fluid temps. I have a temp gauge sitting in my shop, just haven't had a chance to get it installed yet.
Ultimately, I will probably change my radiator. But until then, I will run bypassed. Have been doing so for 3 years now, put on over 110k kms, tow a humungous camper regularly, and have had no issues at all. Proof is in the pudding!
t