To my mind there is no excuse for this appalling treatment by UA. It's no good saying that you should have looked at seat maps if checking in online, or you should have harangued a Gate Agent. Why should a non-regular customer of UA know to do such things? It seems to me that it's up to UA to pro-actively provide the service which the customer pays for - and in this case, recognise that the service is slipping.

If ever there were a worthy case for compensation for being messed around, it would be here - but it ain't gonna happen because the OP is not pushy enough, and that's the only thing that UA responds to.