No conundrum for me. I take the middle exit all the time, and especially the bulkhead. This happens to me occasionally because of last minute planning or change of plans. I don't make apologies if the seatmate feels that their personal space is invaded by my taking the middle seat. It isn't their space, nor is it my role to make them feel good about themselves.

Conversely, I find no material comfort with an aisle seat in the back. Either it's dealing with the seat back being reclined onto my lap, needing to get out of the seat in order to let the window or middle seat pax use the lav, or being brushed by the beverage cart and everybody else passing through the aisle, the aisle seat in the back can be very unpleasant.