I continue to hate most everything about the Klingon storyline. Nothing about it plays well with what we saw on ENT or TOS, and it is supposed to be in between them. To be honest, the Klingon storyline (minus the FED bit) would have fit perfectly into the narrative about the Hur'q, and how they were invaded, subjugated, scattered, had to pull themselves together, etc. Doesn't work with our timeline, but that's where that story would fit for me in isolation.
As for the mirror universe bit? most of the rumors/guesses feel pretty likely, so will see where it goes. Agree that the concept falls apart when looking at it, so don't pick the scab. You can KINDA have a mirror universe episode as a one-off, but it's tough to revisit in different time periods. if the histories are that different, things will diverge more and more over time. It could be CLOSE in the ENT timeframe if the divergence was recent, but so many years later in TOS, shouldn't be anything near similar. Add another 80 years, DS9 would be completely unrecognizable.
But it's really just evil twin cosplay, so not going to look too closely at it...