I think the Asur can't afford to be pissed off at the Empire, and that by preventing a trade deal with between the Empire and Naggaroth they'd gain more than they'd lose.
Pretend you are the Phoenix King. An ambassador from the Human realms has approached you, and informed you that your traitorous kin have begun making diplomatic and economic inroads through the Old World—territory that you consider to be within your sphere of influence.
However, the Ambassador states that they are willing to throw the Druchii under the bus in favour of improved relations with the Asur. How much are you willing to pay to make this happen?
Codes for waystones? Dirt cheap. The High Loremaster? He's travelled through those lands before, and would probably be excited to go back. Military assistance? The last time you landed an army on those shores to hunt beastmen, it didn't go so hot, but maybe with local assistance you can avoid repeating those mistakes?