Shatnerverse basically states the Mirror Universe split occurs when the Enterprise-E time travels chasing the Borg tryng to Stop first contact and pretty much decides the best way to stop the borg in the future is to get Earth in a frenzy in the past about the dangers of the borg.
So when the vulcans arrive for first contact it all goes to crap like seen in the first contact scene for Enterprise.
It would be Cool if the Borg of the mirror uni were all like 7 of 9 more human with a touch of cyber and all Hot Hot Hot, they Invite people and they accept saying "well I guess resistance would be futile"
IMO, Mirror Borg should be an "Individual Collective", where instead of every mind fuzzing to white noise at intense levels, they should be treated as a kind of techno-telepath... where they can block unwanted information. Instead of One Mind/One WIll, it's Chaos in that it's like every other civilization out there. I'm thinking like our Earth in 50 years where everyone has WiFi embedded.