A request for clarification on why they refused was posed to a Chrome engineer in todays Blink Q&A (http://www.youtube.com/watch?v=TlJob8K_OwE#t=13m34s), and according to him the request for integration came shortly after Chrome was released, and the reason for their refusal was the sheer scale/complexity of the task.
In light of this, your initial "if Google had only upstreamed their multiprocess support...we almost surely" and reiterations on this point within the thread do seem a bit like PR sleight of hand, since out of context it implies willingness to do so was the only issue on their part.
1) The answer wasn't "we'd like to do this but we're super busy right now, how about later" or "that's super complicated, will you guys put in a lot of the effort". It was a pretty direct no. We would have been willing to do much of the work.
2) My recollection is that we talked about it around a year after Chrome was released.
Chrome Beta release date: September 2, 2008
Date of WebKit2 announcement: Thu Apr 8, 2010 (after <1 year of development)
I don't have records of the meetings where we walked though.
3) Does the reason for saying no affect whether our choice to make our own thing was reasonable?
With regards to 3), yes it does affect it since a "flexible/malleable no" is an altogether different constraint from a "solid no", so the solution would be measured against a different yardstick in the former case. Pressing on with your own thing in the interests of time to market (and thereby further cementing the more mature implementation not being integrated) in that scenario does strike me as less than ideal/short-sighted.
This is somewhat moot with 1) and 2) being the case (or at the very least strongly perceived to be the case on your side). At any rate neither side being able to settle on a single version of events signals a communication problem, which makes the whole value of this hypothetical joint undertaking fuzzy anyway.