I can't remember who mentioned it, but isn't the portal pretty old? If so, it might've been so long ago that it was either already there when the Vanias acquired the land, or they set it up to have easy access between the Chloas but left it alone as time went by. Domi definitely was only learned enough to make a summons with it, otherwise he'd have been able to account for the connection to Noo's residence causing interference with his ritual.
In regards to the summons, there's a point of ambiguity I can't clear up: who is considered to have summoned Noo? Considering how both Domi and the sixth summoner were directing the rites, and doing it right on Vania property bordering on Gatastate, what is the world's general consensus of the situation?
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Given the Nihtlings' interest in monitoring Noo, them being the sixth summoner could mean they had already tried to curry favor with her (but she seems to have rejected them already, judging by her reaction upon being summoned). Plus, getting a non-affiliated magician, someone as prestigious as a Von Slaught at that, probably meant throwing off the trail to cover up Gatastate's involvement.