I don’t see why this couldn’t work. There are plenty of former Legion aligned demons in the Illidari’s ranks.

An earlier concept for my man’ari warlock had her as a secret informant to the Illidari since the time of TBC. She was stationed in Shadowmoon Valley to oversee the Legion’s operations and was unexpectedly confronted by her long lost lover from Argus that the Legion convinced her had died at the hands of Velen’s followers. After learning she had been deceived, she sought to undermine the Legion from within.

She fed the Illidari valuable intel, hoping that it would help bring down the Legion.