Sorry to hear about your poor experience, that's definitely not good.
Technically you're not entitled to anything. Seats are subject to change at all times, and you are only guaranteed a seat in that cabin and not in that specific seat.
At the same time, it's absolutely unacceptable that the GA acted with such disrespect. The problem is that it's not an easy situation for the GA. They can't out the FAM, and at the same time, there is no way to deal with the issue without lying.
The reason you could select the seat to begin with is because FAM's just show up last minute often. They don't book 11 months in advance, and I am sure UA dislikes this as much as you do.
If you write UA you'll probably get compensation of some sort, but it's certainly not a "right" in this situation.
That being said, I am surprised they chose row one. They are always in row two, AFAIK, since they need to store their "bag" under the seat in front of them.