In the event there was severe turbulence for an extended period of time, what was your plan? You'd need to remain seated in your assigned seat with your seatbelt fastened for an unknown duration

Perhaps the Air Canada Medical Desk could have helped accommodate you prior to travel by allowing you to purchase 1-2 extra Y seats so you could spread out, whenever possible? (I'm not even sure if your requirements would fall under their scope, but it could have been worth a try).

It's a tough situation for everyone involved, your discomfort inflight and the flight attendants being in a difficult spot. If they let you sit down, what if someone else with sciatic pain also wanted to lie down? What if there's someone else with another equally pressing condition who also wanted extra space? Would they disrupt you midflight to ask for their seats back?