Amelanchier is a Ljosalfar prince who has sworn fealty to Arandel Phaedra. He does not lead a separate civ. He was put in charge of the nations rangers; essentially he is the nation's greatest military commander. Arendel still leads the civilian government of the elves, but in times of war may leave most of the important decisions to her most trusted generals. It is also quite possible that Arendel's role is mostly as a ceremonial figurehead, while nobles of the Summer court (like Amelanchier) hold most of the real power.
Thessa is unrelated (as far as I know). She too almost certainly has sworn loyalty to the Summer Court. She is the elven nation's highest ranking Archmage. She probably has more true power than her Queen, but she is not a sovereign.
The Paladins were probably either Bannor or from some civ not included in FfH (technically the elves and the Bannor were on different planes in Kael's D&D campaigns, iirc). There's were not the type of tactics I would expect from the Order of the Elohim (an Order of Monks, not related to the Order religion). They would not try to destroy creation in a fight to stop those who are trying to destroy creation. The are all about protecting the weak and probably get along fine with the elves, considering how Sucellus himself tasked them with the care of his holy sites. (The Elohim aren't really an Order oriented civ at all. Their sphere is spirit and their goddess is Sirona. Of the religions in FfH I'd say their favorite should be the Empyrean, followed by FoL, followed by RoK)
There may have been diplomatic repercussions of Amelanchier's actions, but then again I don't think anyone escaped to tell their nation about it. It is probably assumed that they dies in battle with the demons.
