DA22 : Maybe it isn't proof, but none of the "poisonings" nor other things Nadia did prior to the fight in the big warehouse island place seemed designed to kill anyone. Even after leaving them to die, Nadia didn't seem overly surprised that MC and Leah made it home in one piece. Just another test, perhaps.
Even after Nadia was presumed dead, no one seems to have removed her as an Authority or revoked her commission. As she outranked Leah, and officially still does it seems, according to the rules of the Island Leah has no option but to accept her ranking position -- even the MC is governed by the island's rules.
GD himself may have his own reason for wanting this to play out. It's not like he's always forth coming about his motives for doing anything. On the other hand, Nadia's motives are pretty transparent -- well, now that she's past the slinking around trying to catch the enemy's spy. That was what all the poisoning and attacks were about, if you remember.