I don't see why the CX fee would be relevant - it's whatever fee QF would charge since that's the airline you'd be checking in for.

Personally I have never been asked for a "revalidation fee" when turning up for a flight on a different carrier from that on the coupon (most recently BA instead of AA at LAX last December, which was long after BA has its GBP25 handling fee). I would be absolutely furious to be charged again for using a ticket within the terms of the contract.