yo5454

Hi, my 2017 S550 went through transmission fluid service from a independent service technician trained from the dealer.
2 weeks after the service I got the check engine light with OBDII read out as "transmission fluid pressure sensor" failure.
The car driven totally fine for couple years.

So I bring the car to the dealer thinking it's a sensor replacement. The dealer got back to me that the "valve totally messed up", and it will cost $4k to replace the valve. The diagnosis costs $350 but can't provide me the photo to prove where is the issue.
Questions:
1. Would transmission flush mess up the valve?
2. If valve fails, would I be able to drive couple months without feeling any issue?
3. Is $4k reasonable? Or should I go back to independent guy to fix this since it's identified?
4. Should I get 2nd opinion on this "valve issue" , is this only diagnostic only possible in the dealer ?

Thanks for any feedback!
YO