I'm going to try and answer in a general way such that this could itself be a canonical Q&A, if you had asked "How can we create generalized, canonical questions out of a series of localized ones?"
Can we have a single, canonical question...
Some people write their own Q&A when they notice something that hasn't been covered, which is great. I think about doing it all the time WRT things that have been covered, but (as with your example), not in a clearly canonical way. Stupidly, I still have yet to actually do this, and instead continue to type out answers to questions that should simply be referred to such a canonical version. [Maybe the game point system encourages this inadvertently?]
I have not looked at the actual Q&A's you reference, but if you think they could be summed up -- and of course you are free to refer to and quote them -- then I think you should go for it.
that we all agree to flag duplicates against?
Without a formal mechanism, that's a tough thing to accomplish.1 And although I do empathize with the desire for such a mechanism, I'm glad there isn't -- in essence you'd then have a system that doesn't need "us" to agree to flag anything. You'd just have one person consult a list that was previously agreed to and mark the duplicate, or whatever. The problem with that for me is I've seen questions closed inappropriately (IMO) on S.E. by five people. So either it takes 5 people to agree or just 1 or some other number...and/or X number of people agree to abide by a standard previously set by X number of people.
What we've got already is a sort of a democratic, decentralized "free market" system. No doubt this is laden with inefficiencies and ambiguities by nature -- e.g. I don't know what the heuristic is for the "Related" sidebar (presuming it includes votes, all those inefficiencies and ambiguities are also included), but it can't be made perfect, so it sometimes disappoints.
I can't recall seeing it done, but personally I have no problem with someone writing a canonical Q&A in a case like this and explicitly referring to the not-so canonical questions they are trying to encapsulate. In other words, literally list them like you've done here and then say, "I'm seeking a canonical answer to sum up all of these", then write what you think it should be. Anyone who sees that in future will get the point and, if you've done a good job, support your efforts by using it as the canonical reference. It will also help people looking for answers recognize what all those have in common, and hopefully why yours is the general answer.
1 Gilles mentions the reference question, which I was unaware of. I'm not sure if that can be applicable in all cases though.