All this strongly suggests (to me) that Spock discerned something was drastically wrong pretty damned fast.
Yes, that's pretty much what I've concluded after many, many rewatches over the years. We see Mirror-Kirk thrown into the brig what, 15 minutes into the episode, if that?
If they'd wanted to handle it differently, though, it would have been a great candidate to expand into a two-part episode...one focusing on the regular crew in the mirror universe, and the other vice versa.
That's pretty much what I would've loved to have seen. I just love that episode so much that I want more of it.
