Is it possible that the DD check-in opens after other check-ins so actually you were further down the queue than 4th? If not it seems a strange way for any system to behave to say that you get no benefit from either doing it months before on line or turning up early!
Either way it doesn't excuse a shabby attitude on the part of the agent. Also, to begin the interaction in such a way seems frankly bizarre, and certain to colour the entire transaction.
Perhaps they were on loan from immigration
DD check in opens either 8 or 9 am I think. So technically after OLCI but before the main check in at MCO.
I think that an honest explanation of why the seating requests had been changed in a professional and friendly manner would help.
I guess if the reason were because someone "more important" wanted the seats that might not help. But if it were to allow, say, a family to sit together, or someone with a mobility problem or the like to sit with a travelling companion, it could be more easily understood.
When we were moved last, I couldn't see any obvious reason why the passengers were in "our" seats. However, as has been said before, maybe they just asked for them and as we hadn't turned up yet were given them.
Does anyone know within what time frame this would be allowed to happen? Would OLCI or checking in at the 3 hours prior increase one's chances of getting the chosen seats?