There's nothing the airline can do about the situation after the fact. By all means complain, but at best you'll get air miles.
If the armrest won't go down you must speak to the staff immediately. The armrest is the seat divider. It must be down for take off and landing, except in the case of a passengers size keeping it in a fixed upright position. As in when a passenger has purchased two seats, the armrest would be left upright. By not mentioning prior to take off the crew would have assumed you were happy with the situation. They don't know you're not friends travelling together, and in all honestly, they're not going to go out of their way to check.
Once you've taken off, and if the flight is full, there's nothing to be done except what you've described.
At my airline, we've staff at the airports trained to deal with these sort of situations. They have ultimate say in what happens. If the flight was 100% full, volunteers would have been requested to get off and travel on a later flight. They'd have been compensated for this. The woman would be moved to a seat with a spare next to it. If no volunteers, the woman would have been asked to leave, generously compensated, and put on the next flight. Every effort would have been taken to not embarrass the woman, but ultimately if you don't fit in the seat, you can't travel.
Economy seats on all airlines are much of a muchness. You know if you're going to fit or not. I agree people should purchase the seat that fits their body. Be that two economy seats, or a larger, higher class seat further toward the front. But people don't want to spend extra, so make others around them suffer.