Non-Rev travel isn’t free.

Its a negotiated benefit given in exchange for lower wages.

As for the OP writing in a letter - go ahead, the company won’t do anything about it - you voluntarily agreed to a seat swap, back to your originally assigned seats, but now have a problem with it because the pax didn’t get up to go to the bathroom [did they have nausea when the doors closed that subsided?]. Re-read that and take in how it all sounds. Then do yourself a big favor and just move on.