Well then.. Some more thoughts..
"During winter with the stock cooling setup the TCC releases and engages every three miles.... too much cooling."
That would indicate that the coolant temperature, at the bottom of the rad is below 10 C., as that's the ( at least on mine ) set point for TC lock up. Even at -35 C., mine will stay locked once it's warmed the fluid to that point. My aux cooler is thermostatically controlled, and I have left it hooked up through the winter. The exception to that is, for whatever reason, it doesn't want to stay locked when I'm using the cruise control. Weird, I know, and it drove me crazy until I figured out that the intermittent lock up was only occurring with the cruise on. I haven't checked to see if it still does it now that the weather is warmer, but I definitely will now. The other thing about your lock up coming and going, when you say "with the stock set up", do you mean that you bypass the aux cooler in the winter? The reason I ask is that if the fluid is passing through the thermostatically controlled aux cooler, the coming and going of the TC lock up would seem to indicate that there's at least some flow still going through the aux cooler.
"I have an aux pan temperature gauge - no dead zone."
I was referring to the dash engine coolant gauge. Most SVXs' have a "dead zone" where the gauge sits dead still in the middle unless the coolant gets very hot. My sending unit threw a hissy fit and started reading way low, but it now no longer has a dead zone. I correlated it's readings against the readings from the OBD2 sensor, and now know what where it points to actually means. I was surprised how quickly the coolant temp started to rise in stop and go traffic, as opposed to sitting rock steady at 180 F at highway speeds. My point about that relates to what you noted about the rad flow. If the engine coolant temp is high enough that the fans are on, the T-stat is open, the flow is increased, and the bottom of the rad no longer has an as significant temperature difference to the trans fluid. At least, that's my hypothesis ..
"I also have a thermostatic bypass valve, which redirects all fluid flow back to the trans - starts to open at 160F and is fully open at 180F."
That's the other part that makes me wonder if the thermostatic valve is working properly, because of the intermittent loss of TC lock up in the winter.
( Infra red thermometer + GoPro + smart phone App = instant remote, real time telemetry )
Duct tape, zip ties, and other implements of security may be required. If the first end tube on the aux cooler show 140 F when the line only shows 150, it's not closing tight. If the first end tube on the aux cooler only show 140 when the lines show 200, it's not opening properly. Those numbers aren't exact, but I think you get what I mean..
Best of luck, Sir..