Low season huh. Anyway, your answers are good promotions. I even suggest that you write a Volvo service manual. I found the Haynes 850 service manual contents mostly sugesting the reader to leave it to Volvo dealer. The book doesn't even tell you where to find the relays and what are they for.
As you already know, the transmission is Aisin and so the ECU. I don't know the ECU price, but $ 35 sounds cheap to me. You are right about ECU faulty possibility, but simple cable problem is also possible. Now its a matter of replacing the parts piece by piece till I goin broke.
Me mixing the cooland and transmission fluid? or you are telling me that there is a leak between radiator and transmission cooler (which I doubt that it can happen). No, the coolant is green and the transmission fluid is red.
It's been a week I drove Japanese, and leave the Volvo polished by my son.