First thing I would rule out is to see if you have the infamous RMS leak because sometimes oil leakes from the seal into the clutch/transmition making it slip, and in that case it would be covered by the warranty. Just a thought in case you haven't already done so

EDIT: Also there is a TBS on faulty trannys in which they pop out of 2nd gear to neutral all of a sudden by itself which I don't think is related but just in case, has that ever happened? that would be covered under warranty too.

All I know is that needing a new clutch in 20k with normal driving sounds very fishy to me, there has to be something wrong...