Primordus is just a walking volcano. I don't remember anything mentioning he was the greatest Elder dragon. Jormag staying back could be explained by knowing they are each other's weaknesses and only fights Primordus in the end as a last ditch effort cause Ryland isn't winning. Primordus can be explained hanging back because Braham is its brains.
Eir's bow served it's destiny. Had it not been stolen Braham would have never become the champion of Primordus.
There were consequences. Although Aurene was able to deal with alot of the energy she says a bunch of it went missing. And we can't be sure how long it takes for leylines to overcharge. It's possible Aurene might not be able to unclog the drains forever on her own. The Elder dragons job is to just absorb a fuck ton of magic so I assume the planet doesn't explode. As long as you got a good dragon doing the job there isn't really a point keeping the others alive if you want to keep your lives.
Pretty sure you see Tengu and Koda in the mission. Maybe who shows up is based on who you helped during their events?
I'm not a fan of the dragon response missions and I don't see many people will be doing the content 4 years from now unlike map metas. Even the masteries are worthless for any content besides dragon response missions. I haven't done all the old living world stuff so there might be some other masteries just as useless for the rest of the game but I haven't seen any yet.