They don't. Seats are not airline tickets. As proof: airline tickets (or reimbursement) are guaranteed; seats are not
Exactly.
The closest I could get to any impropriety in giving away your upgraded seat would be if you then claimed EXP benefits in the back having 'upgraded' a non-elite person into your seat.
Not of course an issue if you are both EXP as would be the case for you brp