Really? I haven't flown as much as you, but I'd move the armrests after the pre-flight cabin check or right after take-off and never had anyone mention say anything or make a face like they noticed. I think flipping down the table would be obvious and not recommended.

Regarding the OP's question I'd probably allow the person into the seat next to the other person, or even the one next to me if it my own seat wasn't fully functional.

Generally flip down the table on the free seat so it's not instantly obvious it's free. I've had someone try to get in a blocked seat after boarding was completed on continental flight and I flat out asked him if 7E (or whatever it was) was his seat, he tried a different row. A fellow pax wouldn't know if I bought a second seat for myself - I'm chubby, but not "horizontally challenged".

I think it's awkward when seatblocking happens in C on longhaul and I'm told of it by the purser, other pax give me funny looks when I eat in one and sleep in the other and almost every other seat is taken... Haven't had any issues, but if you told someone the seat is broken they'd likely believe you if he or she is a regular LH flyer.