I'd be curious to know the fare bucket availability in the 1-2 hours before cutoff. Its always possible that 2 last-minute first class tickets were sold, especially if there were discounted F buckets available but only very expensive Y buckets, but Shena is also possible here. Is it possible for the SC agent to know that 2 last-minute F tickets were sold? I'm not sure.

Either way, I agree that you should get your original seat back. DL inconvenienced you by kicking you out of F, but somehow it's impossible to inconvenience someone else to give you your carefully selected seat back. In reality, it is possible for the GAs to do this, they're just not interested in helping you out. Another DL service failure that seems to be more and more common lately.