On my first viewing, I skipped ahead to when they are back on Dannus V. I didn't see anything canon breaking in having the GoF having an avatar. They were acting out of self-preservation.
On my second viewing, I watched the whole episode. I don't care much for the mirror universe. I think that the Emperor's actions created an alternate timeline, a parallel universe, so what we saw in the first season still happened.
I think it is the first time since Enterprise that we have had a reference to Denobulans.
I noticed in the second viewing that the bracelet on the Emperor's arm had a red light, not a green light, indicating her death was imminent.
And, I think this is the first time that planet in the mirror universe is actually different in appearance from its prime universe counterpart. Risa, in the mu, had a ring system, while the Risa, in the pu, does not.
I was disappointed in the beginning credits. Flipping the images upside and giving them a different color hue is, for me, a "why bother?" thing.
Finally, are they setting something up with Booker? The writers are creating the aura of suspicion about this character.