The newer Sherman/Shwartz books describe pretty much the same exodus, but the actual departure is observed from the POV of some relatively minor characters and involves much infighting on Vulcan, with Surak playing only a minor part and S'Task acting mostly "off camera". The departure is panicky where Duane describes it as almost scholarly; also, some elements of the journey as described by Duane are made to look like misunderstandings/exaggerations committed by later history writers, whereas the S&S books describe the "ground truth" of how it really happened. The differences between the stories are minor in the end, though.
S&S omit any mention of a Duanesque alien invasion attempt as a catalyst for the exodus, but the stories certainly don't preclude the possibility of one. Vulcan might have been subject to many an alien visitation in both the S&S and Duane pasts.
...perhaps explaining some of the other Vulcanoids found in TNG's time?
The Debrune of "Gambit" fame were explicitly described as Romulan offshoot - and supposedly our heroes did know what they were talking about. If they were "leftovers" from the original convoy, then this would mean our TNG heroes consider "Romulans" a population group or culture that predates the settling of Romulus. They could also be part of a second wave of migration, out from Romulus, and perhaps back towards Vulcan again; Duane describes the loss of starflight capability and ambition on Romulus soon after arrival, but that, too, may be considered mythology rather than "ground truth", perhaps an excuse invented by the more modern Romulans to explain certain failures in their past.
I though P'Jem monastery was founded as a place for Vulcans to purge their emotions. Why would they do follow this paths centuries before Surak came along?
The monastery could have changed hands at some point, much like Hagia Sofia is a mosque (or rather a mosque museum) now despite having been built for a different faith. Or then Surak didn't invent the emotion-purging movement at all. It does sound like a natural thing to think of, in a society torn by emotion-driven war, now doesn't it? Surak might well have had like minds preceding his ideas by thousands of years.
OTOH, for all we know, Surak never actually was for this emotion-suppression thing himself, and merely promoted the use of reason. Various preexisting political or religious groups could have adopted Surak's political success as their own, adding their own, sometimes alien flavors to the movement that rode on that success.
What we know of P'Jem is this:
T'Pol: "It's an ancient spiritual retreat. A remote sanctuary for Kolinahr and peaceful meditation."
T'Pol: "The temple is almost three thousand years old, Commander. You can't expect it to be in pristine condition."
The temple is a Kolinahr retreat now, and is three thousand years old. We don't know what it was before, and it's possible that it wasn't even located on this remote planet originally - but from the context of the second statement, we do know that the actual physical building is that old, and not merely the institution.
Timo Saloniemi