Thanks for the chance to assist
I can not imagine any circumstance that would allow radiator fluid into a transmission absent mistake by someone who is working on the transmission.
If the body shop was overall responsible for the repair, they are the one that needs to fix this problem.
They are the ones you will need to apply pressure to.
I would write a demand letter, a simple letter in which you lay out the problem as you see it and demand they (body shop) fix this problem and if they do not you will sue them.
If they do not solve this problem, you can sue in small claims
You may need to take the car to another shop to get it fixed...keep the bill as you will need this to prove your case.
As to the rental car, you need to return that...but if you need to rent another car, make sure you keep those receipts as well
Threaten the body shop with suit if they do not fix this and then, if you need, follow through
Please let me know if you have further questions; if so I will do my best to answer them. If not please hit the accept button, its the only way I get paid.