So my TC is likely bad, however I am going to try and do a fluid swap. I can find a pentosin fluid locally for the $15 a quart but why should I put that in????

It is clear that many TC's failed failing throwing a code. Maybe its the seals, and maybe my valve body and trans will follow in the future but here is my question. All of this is happening with the factory fill pentosin. The pentosin is not protecting the seals. Its not protecting the valve body, how is another "compatible" fluid going to make things any worse?

I dont think it can hurt. I have done swaps on other Euro marks particularly BMW's with non factory fill and they run fine. I have yet to hear of anyone who used non pentosin sub and had a slipping issue due to the fluid and its likely if you did, you already had problems.