LOT Polish Airlines  -  Service on board

I would like to share the experience I had with LOT travelling from Poland to Toronto, Canada. I was on flight #LO45 on the 13th of October 2019, and I had bought my ticket two months in advance AND I checked in 36 hours prior to the flight because I have a problem with my knee: for comfort I was required to keep my leg straight for the duration of the flight; therefore, I reserved seat #28H.
When boarding was complete, the flight attendant approached me and said: Madam, please move to seat #21H. We need give your seat to someone who had purchased their ticket before you did. If you refuse to move, we'll remove you from the plane. I tried to explain the situation with my knee, and I wanted to verify when the other person had bought their ticket (I had purchased mine two months before the flight) but nothing helped. She kept raising her voice (and was, in my opinion, somewhat rude). To avoid the situation getting out of control I moved (as requested) to the other seat.
1. I wonder what rules and protocols does LOT have that allows flight attendants to move people from a seat reserved thirty-six hours in advance on a flight bought two months previously to an unsuitable location so that they can seat someone close to (in my opinion) a friend or relative, thus ignoring the needs of someone requiring a suitable seat location for health and comfort?
2. Also: can a flight attendant refuse boarding to someone even if that person is in possession of a ticket purchased two months previously AND then having reserved a seat thirty-six hours in advance?
3. If this is the normal LOT practice to remove people from the plane for no reason would LOT have provided hotel accommodation if another flight couldn't have been booked until the next day?
Thought I will never file complaint or send letter to anywhere but two weeks passed and I still remember that angry attendant and her voice and all the details. It isn't right, nor fair, that someone can be moved around an airplane after booking the ticket and then checking in far enough in advance to be guaranteed a comfortable flight.