I agree entirely with your rationale and choice of seat and glad it worked out, but you seemed to have been lucky in the FA not asking you to take seat on BP (a call notwithstanding) AND presumably the gent had a BP with 2D on it.
Did he do the "Excuse me, I think you are in my seat" ? If not, then it definitely sounds like someone arranged a favour which was undone when you objected.
BTW, did you have a paper BP, or just the app? If you had a paper BP sating 2D I wonder if they would have tried it on? EDIT: I see the answer in post above
. Paper BPs are STILL always a thing for me!!