The warrenty void issue is concerning the Samsung side, not tmobile side.
In the majority of cases tmobile does not even care or check if knox has been tripped, but on the samsung side if, and they normally do from what I gather and have checked into, check for knox, then they will simply refuse the phone and send it back. I've talked to numerous people that this happened to so I know it does happen.
Sending into tmobile I seriously doubt they will even check to see if knox has been tripped.
Sadly though this part of the phone cannot be reset, not evne useing triangle away. This is why when you see a rom it says somthing to the fact of "You warrenty is now void" this is referring mainly to the samsung side warrenty.
As long as you flash a stock firmware onto he phone tmobile will swap out your phone for another.
You can grab a stock firmware file from sammobile and flash it via odin and that will set you back to pure stock setup, just like when you got the phone, aside from the knox trigger. I'd leave it not setup at all, this way they can do the setup if they want or need to Just tell them you reset the phone to factory default, don't mention the fact you have rooted it or flashed it or anything. My guess is they will never ask and will never know.