Let’s get to the root of the matter. The OP bought two seats and selected 18EF. 18D stays unoccupied, OP feels they have first claim to seat. Non-rev changes to 18D claiming need to use bathroom: OP observes non-rev not getting up during flight. Ok take out non-rev and insert non-elite....we’ve seen this entitlement theory over and over again. The question then becomes who really has a claim over the unoccupied seat...what if the situation was reversed. Sorry, frustrating as it may be, I don’t really find myself being all that sympathetic....I try and avoid these types of seating scenarios by booking myself and travel companion in window and aisle with the hope that the middle will ultimately stay unoccupied