P0578 error code on 2003 avalon with brand new transmission installed

Question

Hi I have a 2003 Toyota avalon xls, with 93000 miles. I purchased the car from a reputable dealer, although I now believe when I purchased it, there was a hidden error code regarding the transmission. A few months after I purchased the vehicle, I needed to replace the entire transmission. I ordered a remanufactured transmission from a reputable company and hired a transmission service company, who have been in the transmission business for over 20 years to install it. The transmission company had my car for approximately 3 weeks attempting to correct the error code p0758. They were very shady in their communication with me and i'm inclined to believe they know more than they were telling me. The first thing they said to me was that the transmission went bad because the radiator was bad and allowed radiator fluid into the transmission. I asked them if the new transmission would be affected in any way due to the radiator fluid leak and they stated no. So i purchased a brand new toyota radiator for $300.00 and had tthem installthat also. Next, they said the oxygen sensor connected to the radiator caused an error code and had to be replaced. I questioned him about the error code, because the car had absolutley no error codes when I brought it to the transmission shop. Next, he said the ABS module had an error code of a short circuit. Again, I questioned how that could happen when there was no prior error code. He installed a used abs pump and module. Getting back to the p0758 code, I did some research online and found that the p0758 code can relate to a bad ecm under the dash, which controls the shifting of gears. I checked the ecm under the dash and it looks brand new. I should also mention that the check engine, vsc and trac control lights on the dash board came on once for about two days and then went off. When I used my meter to check for a code when the lights were off, there was no code only and warning light for a potential problem. My question is this, how can a shift solenoid go bad on a 2 month old remanufactured transmission? and if its something else, how can I trouble shoot the problem to fix it,or at least know what the problem is to have it fixed. I believe my brand new transmission will also go bad if the error code isn't fixed once and for all. Thansk for any help and if possible pictures. Renee