Ruling from Alex Werner, FFG Game Rules Specialist via the official rules question form:
Question: Hello! If during Mythos Phase, the treachery Watcher’s Grasp makes the Spectral Watcher “move, engage, and attack as if it were the enemy phase,” can I swing my Survival Knife (2) at him, which has a reaction trigger of “when an enemy attacks you during the enemy phase”?
Answer: No. Survival Knife (2)’s reaction ability can only be triggered during the enemy phase; Watcher’s Grasp does not create an “enemy phase.”
(So with treacheries like this, which cause enemies to attack "as if" it's the enemy phase, other cards will not "resolve with the altered game state in mind," in contradiction of the FAQ rules on "As If..." This raises the question of whether MJ's ruling about the enemy exhausting would also be invalidated.)