Bloodycelt, I have to say that your pitch hasn't gotten any more convincing to me with subsequent iterations. There are four separate corebooks... but to run what most of us seem to think of as a "standard" l5r game you'd only really need two, because the other two are actually more like splat books for very specific campaign types... so then you've got just two "core" corebooks each reduplicating a significant chunk of the mechanical and setting information of the other... and I'm back to being at a loss as to why they don't make more sense as a single book. (I'm even less certain what to make of the totally different breakdown in the second half of your post, in which shugenja seem to have disappeared entirely from everything except mass battle).
I'll also add that I'm pretty confident I could run the campaign Shosuro describes out of the 4E corebook alone if I needed to. Other books (mostly EotE) might be time-savers and sources of extra inspiration but are hardly integral. That's more than I can say of any of the choppings-up you've suggested across multiple threads.
Well, the Shosuro Actors are a School which is presented in the Great Clans, but if it were restricted to the Corebook I would have played a Bayushi Courtier or a Soshi Shugenja and talked with the other Shugenja or the other Courtier player about how they want to focus their character, so I can focus mine in an other way.
But yes, the Kolat and the Spider are mentioned in the Corebook and regarding the Kitsune-tsuki it could have been created by the rules in the GM-Toolbox from the Corebook. ^^ So this campaign could be played with the Corebook only.
Regarding the idea of making one book about mechanics "only" and one book about Rokugan, this sounds like a nice idea. They could be released at the same time as seperate books or as a bundle. I would rather buy this, than various Corebooks just to get alle the mechanics I may need for my games.