In my eyes the ascians attack doesn't have much to do with what midgardsormr did. He did seal the protection of the main character but that shouldn't affect the headquarter of the scions in any way. Well it's true by doing so he made it visible that the protection was getting weaker and so nabriales found out that HE now can go into the headquarter to. But you should not forget that he claimed that other ascians could already do it but for a reason i don't remember see no reason to try it right now. So even without midgardsormr doing anything sooner or later someone would have tried to take the broken staff and maybe at that time there would have been nobody in the way of the ascians.