Beric Dondarrion has the annoying "cannot be killed" text. So when selecting characters eligable for military claim you cannot select him, which is fine, but also means he cannot be targetted for events that allow you to kill a chosen character. However what about wildfire assault which has the "cannot be saved" text? My opponent chose 3 other characters to be killed, but since Beric Dondarrion cannot be killed, he was given a fourth for free (technically he was not being saved, because he could not die). Is that correct, or because he cannot be killed should he be automatically be one of the 3. Likewise if someone had forced you to keep a character kneeling, and your plot card says "knelt characters cannot be killed", can you select 3 other eligable characters to "save" then the knelt character cannot be killed?

I have also started looking at the Lords of Winter expansion. There are a couple of "stand character" locations. Frozen Outpost says "Challenges: Kneel Frozen Outpost to choose and stand 1 defending character. Until the end of the challenge, that character gets +2 STR (+4 STR instead if it is a War character)." It does not say that this action removes the card from the challenge - so is that character still participating? The Mole Town Brothel says "Any Phase: Kneel 1 influence to choose and stand 1 defending character." Again it does not state that you are removing them from the challenge. Are these just a way to allow the defender to be used twice?

A "cannot be killed" character does not have to be chosen for Wildfire Assault. So yes, any "cannot be killed" characters are extras (Wildfire Assault will fail to kill them).

A character is removed from a challenge in 3 circumstances: (a) it is specifically mentioned on the card (this doesn't necessarily stand the character); (b) it leaves play; © its controller changes (FAQ section 3.30). Merely standing a character doesn't remove it from the challenge (yes, it is a way to use the defender more than once).