If you're facing a loss against someone with more than 1 claim on his plot for the turn, are the deaths simultaneous? a.k.a. Will I have to target Aeron Damphair and have him react while someone else gets killed as well or can I have Aeron react, then direct the second kill at the new dude he brings into play?

As stated in the rule book (Expert Rules -> High-claim Military Challenges), all characters chosen for claim die at once.

Thanks, that clears things up.