Sounds like quite a nightmare.Sorry if I missed it in your post,but did you book through a travel agent by any chance? I have seat assignment problems every time I use our corporate agency.My assignments were changing several times a day-each time I'd call UA,they'd tell me they had no explanation.After several days of this and countless time on the phone,I finally got a supervisor who informed me that the travel agency computers automatically search for the "best seats" periodically,several times a day.Once I called the agent and told them to turn off this feature,my seats did not change again.Unfortunately,the seats the computer sees as better,are actually worse in my opinion.

I wish UA would allow us to change seats online-it would be less stressful for the pax, and certainly less labor intensive for UA each time they need to deal with seats on the phone.