I like how you take the extremity of the Mirror Universe and justify it rather than trying to write around it. Trying to describe or depict the MU more sensibly than it was often shown on screen might have entailed uncomfortably ignoring its exaggerated aspects, but instead you've taken the approach of searching for explanations. Given that the wormhole can lead to the MU, it does make sense that the orbs are involved.
I also like how even Sisko didn't acknowledge the Kai's name, as if the Kai has been blotted out of not only history but the existence of the Celestial Temple, too (other than in general awareness of what happened). It suggests almost a humility on the part of the Prophets, an unease in that while they're quite aware of their (potential) error, the exact details are not something they want to consider. The general knowledge and basic awareness is enough; maybe safer, too? Tying in to that, I also like the explanation for why the Prophets - timeless beings - would only begin sending orbs in the last 10 millennia. They've already rendered a whole swath of Bajoran history too satuated with dangerous knowledge or understandings to justify the risk of contact. I wonder if to non-linear beings, that would be like having patches of your mind or awareness traumatized? I like that the Prophets have a sense of caution, but it makes me wonder if to them that stretch of Bajor's history is like a negative equivalent to what Dukat ends up with - fuzzy, but more like a guilt-driven bad dream or a sickening awareness than a vision? Anyway, it seems to me that there are hints of shared experiences for wormhole aliens and mortal beings, here.
The short explanation of the whole branch of the multiverse corrupted by the Kai is great, because it focuses on the actual implications, the terrible waste and the tragedy, rather than just on the events themselves. And of course your choice of historical focal points reflects that, with people like Saul and Washington failing to take the path they did in our reality - the true evil of the MU is not the creulty in and of itself but the fact that noone seems to have the strength to turn around and try to reverse or stall it.
As a final point, I particularly liked how you acknowledged the Ferengi's MU portrayal and noted that they're one of the few hopes for that universe (even if it is a very slim one). It leaves me curious as to how they ended up not only one of the few bright spots but probably a more "enlightened" culture than they are in our universe. It's a shame we don't know enough about Ferengi history to guess what happened to send them down different paths...