In the episode In a Mirror Darkly II, Mirror Archer is apparently experiencing visual and auditory hallucinations of Prime Archer. It struck me today that perhaps both Archers were born with schizophrenic genes, but only the Prime version had it treated as a younger person. Then again, the recent DSC ep 3x04 (not a spoiler) suggested that perhaps mirror universe humans have a variant genome predisposing them to be violent lunatics. So maybe it is unique to Mirror Archer after all.
Maybe it runs in Archer's family, his father was also mentioned as having suffered from and eventually dying of mental illness as well.
Mirror Universe Archer cannot admit he has a mental illness, and will not accept help or medication, otherwise someone will make him their gimp. Prime Universe Archer pops a couple pills with breakfast and he's fine. Henry Archer never had to "invent" the Warp 5 Engine. They Killed a lot of Vulcans and took what they wanted. The Terrans had warp 5 Star starships by 2065.
Clarke's Syndrome was described as a degenerative brain disease, not a mental illness. A couple of the symptoms mentioned, hallucinations and not recognizing family, are consistent with dementia, so I assume it was more like Alzheimer's, but with terrible pain too. I think Mirror Archer was pathologically insecure, paranoid, with delusions of godhood. Reminded me of Garth of Izar. But Mirror Just About Everybody was kinda touched in the head. Like there was something in the water over there.
It's metaphor for Prime President Admiral Federation Founding Archer casting an impossible shadow over Mirror Archer. A made up voice in his head, rather than a literal hallucination.
In all likelihood, it’s this. Federation Universe Archer only suffered confusion in regards to contradicting dates when Henry Archer died. And its not really confusion if Archer entered flight school at 12 and then dropped out and did not come back until years later to graduate at 24. So while one could write those inconsistencies off as early signs of Clarke’s disease, its unlikely. I am convinced that Mirror Archer's mental issues are unrelated to what happened on the Defiant that affected the crew that caused them to go mad and kill each other. We only saw signs of madness from Mirror Archer. Since the mirror counterparts of T’Pol, Trip, Reed & Phlox likely did not survive long enough after IAMD to see them affected like Archer. And Mirror Mayweather and Mirror Sato showed no symptoms either. Even when Kirk boarded the Defiant in “The Tholian Web”, he did not go crazy. Mirror Archer made it all up in his head.
“Theorising that one could time travel within his own lifetime, Captain Jonathan Archer stepped into the Quantum Leap accelerator and vanished... He woke to find himself trapped in the past, facing mirror images that were not his own, and driven by an unknown force to change history for the better. His only guide on this journey is Porthos, an observer from his own time, who appears in the form of a beagle that only Archer can see and hear. And so Archer finds himself leaping from life to life, striving to put right what once went wrong, and hoping each time that his next leap will be the leap home…” “Oh boy…”
I did think it would be funny if in that final scene, when Archer is walking up the stairs to address the fledgling Federation, the blue glow of a quantum leap enveloped him. That must exist as a fan edit out there, somewhere.