Personaly i don't see it as a merge.
Apart the fact that it was not "Coria" but leftovers of it that still didn't consist in a realm, the fact Munro was lord of the region gives him the liberty to join with it any realm he likes.
I guess the real question here is if that region would even make it on it's own as Coria realm?Because i doubt that's even possible.All i see is a region with a lord that it's lord decided to add it to a realm, which happens all the time in this game.
Also as far as i know Silnaria sees her self as the real Coria, meaning the other is just regions the real Coria rejected.Not a realm.
All i see is a lord that took his region and joined a Realm, which happens all the time in BM.
