So there was a problem with the seat and it appears that the CC dealt with that by shifting a staff traveller into the broken seat (pretty much standard policy on many airlines) so the OP had a fully working one.

OP moved to the working seat and then slept. Then for some unfathomable reason went back to his original seat and turfed the staff occupant out and then sat there in its broken state for a few more hours.

The wrapper left is a minor issue. I assume the staffer just felt that it would be their seat for the rest of the flight and got themselves comfy. On the OP returning the staffer obviously collected his stuff but left a couple of wrappers there because with the OP looming they wanted to evacuate the seat ASAP.

The staffer must have thought their luck was in when a revenue passenger wanted to sit back in the broken seat and they could go back to the working one

The mistake the OP made was returning to the original seat rather than staying in the new seat.

I really don't think BA can be expected to offer the OP anything substantial (other than if they paid it the seat reservation fee) . It's not their fault the OP did something 99.99% the rest of us wouldn't do and deliberately go and sit back in a broken seat.

Sorry OP you are on a hiding to nothing. You did, once you moved, get the comfort you desired and paid for. Returning to a seat - a seat you knew was broken - was just stupid.