RK, this is indeed a little puzzling, especially when you consider the location of the engines. The assymetric thrust produced by an engine failure with tail mounted engines is much less significant than a similar situation with wing mounted engines (much greater moment of action about the C of G in the horizontal plane).
Would you be able to type the exact wording of the flight manual with regards to the mention of "critical engine" as the context might give us a clue?
My understanding has always been that a critical engine will under all circumstances have a greater effect, whereas in the case of the downwind engine effect it could be either engine.
As always, I stand to be corrected of course. This is just my understanding of the phraseology used in the certification.