Why did you acquiesce and occupy that seat? I would not have under any circumstances. I would have refused to take a seat if the person in the seat next to it was spilling into the seat.

Suggested approach - just get off the airplane and go back to the gate agent, explain the situation and give them four options. They can either A) find me another seat on this flight; B) find me a seat on a different flight; C) find the pax of size a different seat(s) on this flight; or D) find the pax of size a different seat(s) on a different flight.

Since you have reserved an entire seat on the original flight and the airline could not accomodate you since POS was taking up some of your seat, I would suspect that IDB compensation would apply to you if the airline elected Option B above.

Having less then 100% of an entire seat available to you is just not acceptable. But the time to deal with it is before the airplane door closes. Force the airline's hand and make them find an acceptable resolution.