One key point where I do diverge from others at the "year" level: there are fifteen years between "Space Seed" and STII:TWOK — not 18 or any other number. There are at least 20 years between "Errand of Mercy" and STV:TFF, given the history of Nimbus III. Yes, this means STII and STV are more than just a few months apart from each other. A lot of people seem comfortable ignoring this, but I don't, dammit. :-)
More like between "Balance of Terror" and TFF. That's the most intractable data point -- there had been no UFP/Romulan contact for over a century before that episode, therefore no joint UFP/Romulan/Klingon diplomatic project could've been possible for some time before then. I don't see "Errand of Mercy" as a hard
terminus post quem there, since it's possible that the early talks for a "peace planet" could've been underway earlier as an attempt to
prevent the war that broke out there. Although, of course, the
Vanguard novels put its origins after that.
On the "20 years" thing, I never liked the Okudas' insistence on treating every round number as an exact date, ignoring the fact that people round things to the nearest 10 or 100 all the time. It would be an insanely improbable coincidence if not even
one of the round numbers uttered by the various characters over the history of the Trek universe were an exact figure instead of an approximation or an error, so it makes the universe more realistic if you
don't take every stated date as a precise figure, especially the round numbers. And the assumption that some such figures are rounded off makes it easier to deal with discrepancies and unlikelihoods. (For instance, the Okudachron's claim that the
Valiant was launched in 2065, just two years after Cochrane's first flight, is ridiculous. That's a case where some degree of rounding must be assumed, since there's no way it could've been launched that early. The "200 years" figure has to be rounded up from something more like 170 or 180.)
Are there some contradictions between early novels and later canon? Of course. Are those contradictions drastic enough that the details can't be elided and they should be considered part of an alternate reality, as CLB often says? Not to my mind. It's my head canon, and I'm sticking to it!
Well, yes, of course. That's part of the fun of it, the fact that everyone gets to make their own decisions about their personal take on the universe. We can share our own reasoning with each other and debate what approaches are better, but it's just with the intent of exploring the ideas and the creative process of building these chronologies, not demanding that others conform to one's own model.