We (the community) are not sure if the "powerful beings" are actually NPC's.
As you said, we guess they are, but they could also be many other things. New Worlders, golems, or who knows what. PDL (platinum dragon lord) can use wild magic and has created some pretty impossible things according to Yggdrasil rules, so that leave a lot of unknowns.
You are correct about if the guild weapon is destroyed, it destroys the guild. At least that is what's in the LNs, but we never saw an example of this in Yggdrasil.
My personal guess was that you could destroy the guild, then take it over and create new NPC's. It would be sort of a letdown if you capture a guild and your stuck with someone else's npcs. There is probably a way to delete them and gain your guild's NPC levels back. Each guild has a set number of NPC levels, so they are very important to use them wisely.
However... the new world isn't Yggdrasil. The magic and mechanics are very similar, but there are some strange differences. A great example of this is Ainz's death knights. If he uses a corpse they last forever, which wasn't possible in the game. These changes could potentially create unknowns. Maybe the NPC's will automatically disappear, maybe ainz has to kill them... maybe there isn't a way to get the NPC's levels back. Maybe it will reprogram them to follow Ainz, or they will become their own beings and be broken from the NPC system of the guild.
There are so many unknowns that it is really hard to say how it might work. We don't even know if Ainz can create NPC's in the New World. Ainz mentions it requires a dev kit... but would it even work? Man it's a tough question.
I've been hoping that the author writes about Ainz taking over the 8 greed kings base. I think this could answer a lot of questions we have.
- Can ainz take it over?
- Can he move it?
- Can he create NPC's?
- Could he transfer them over to Nazarick?
It endless, but maybe someday we will learn more.