Having done it, the NP205 is really, really easy to disassemble, inspect and reseal. Its very straight forward and youre just looking for any chipped gears. To be ultra safe, just buy a set of good bearings to throw in when you put it back together. The hardest part is lifting it up onto the work bench lol
I would just take the C6 to any trusted transmission shop to have them reseal and inspect.