Hey guys, first time poster for a headache of an issue. I'll try to keep it short, but I could use some advice asap.

During a transmission flush for my '07 328i Coupe (112k miles, hard 2-1 downshifts) one of the pins broke off from the TCM. Purchased a replacement TCM, however, just realized that it came off a 335i. Now the car turns on but shuts off when shifting out of P and into R or D.
I ended up towing it to my brother-in-law's mechanic. But, because he's not a BMW specialist and just a mom and pop shop for all cars, I'm now skeptical about the results. He's claiming the tranny is locked up and wants to put in a rebuild for $3600. He won't tell me how he got to that conclusion, and is rushing to get the car off his lot after a day since we've talked and telling him I need to figure out what to do.
I've also talked to a local BMW mechanic shop in Pasadena, just to feel out my options, and he said he'd want a $600 retainer to diagnose. 50-50 chance he'd figure it out or be prepared to pay for further services ($$$$).

Here's where I could use some advice. Would the fact that the replacement TCM coming off a 335i, instead of a 328i, be the reason why the car shuts off when shifting out of P? If I can recall, 328i coupes have 6HP19 while 335i have 6HP21.
Or, is it just coding issues? I only remember using inpa to change the vin on the TCM.
Would it be preferable to find another TCM, either with or without Mechatronics, from a 328i?
And, if worse comes to worse, could it have been possible for the tranny to "lockup", even though it was just fine right before servicing, needing a rebuild?

I'm in between getting the rebuild tranny, or towing it to a local mechanic that specializes in BMW, or towing it back home to try again.

Any advice will be helpful and will have my thanks in advance.