Why in the world do we need Kirk back in the 24th century? If not for the Nexus, he would've most likely been dead by the TNG era anyway. And it's not like there's any dearth of 23rd-century fiction featuring the character.
King Daniel wrote:
IMO the Sharnerverse was close enough to the mainline continuity that it can be seen as part of it if one so chooses. They just never ever talk about him in the other books.
You could perhaps make a (tenuous) case for that where the first two trilogies are concerned, but the Totality trilogy is incompatible with the main novel continuity in a number of respects: Bajor hasn't yet joined the Federation as of 2378, Titan
's Romulan-aid mission lasts over a year, and Admiral Janeway is alive during the time frame when she was dead in the main novel continuity.
I think that initially the idea was to approach it as you suggest, to leave it ambiguous whether they could fit or not, but by the time of Totality the decision had been made just to let each series go its own way. And as zarkon
says, The Return
's version of the Borg and V'Ger has been retroactively contradicted by the main novelverse.
For that matter, the Shatnerverse version of the Mirror Universe's origin is incompatible with "In a Mirror, Darkly." The Shatnerverse version is that the MU began to form after First Contact, growing out of a human-Vulcan military alliance formed to defend against the Borg threat which Cochrane had learned of. IaMD established that history had already diverged much earlier than that -- the Empire had existed in some form for centuries, most classic literature other than Shakespeare was different, and Cochrane and his people slaughtered the Vulcans who made first contact.