Reg 261 does not apply to an uncomfortable seat! Everyone in economy would be after compensation then as every airlines economy seat is a chiropractors dream.
As long as the seat belt works and the seat is in the upright position then it is a safe seat.
The airlines obligation is to get you from A to B and nothing else. No food, drinks or entertainment are REQUIRED (ok depending on the length of the flight water might be a health and safety requirement)