( Just how uninformed are new individuals to their group? Does Karsa not give a thorough summary to new arrivals? And they are all then expected to influence this city without understanding the implications? )

I imagine you are referring to a different one than what exists here.

( One that he clearly has quite a bit of history with if the reaction is any indication. )

...the Brotherhood is what the alliance of the Undead Lieges refer to themselves as. They have attacked many cities, killed and taken control of many people including the very first to come through the beacons, and taken over a large majority of Akhuras... this world. This is the group Messalina claims to want to destroy so she might free her own undead army.