...did we honestly have unity under the d20 system?
I think the answer here is Yes and No (just to muddle things up a bit

).
I think we did have unity in that the OGL accomplished, at least intially, what Mr. Dancy intended: Unify the market (mostly) under a common mechanical system so that everyone could share the market, rather than continue to divide it (and by divide I mean between different publishers, and within publishers with self-competing products).
But then...Yeah, it most definitely caused even more division IMO. I think that for the
health of the market, the OGL was too open...though I'm glad it exists so that Dancy's and Adkinson's other reason for the OGL could be realised: that there would always be a version of D&D freely available to the world at large, despite what may happen to the brand or it's current publisher.
Additionally, again, if we had unity, why did so many publishers abandon 3e/3.5?
I don't think it was solely lack of unity that caused an abandonment of D20 publishing, I think it was just an inability to make money off of it anymore due to the normal life cycle of game system/products - though the eventual diffusion (lack of unity) of the D20 system through so many different products and publishers is a major factor in that - just not the only one.
Did 4e cause the split or simply highlight what had already happened?
I don't thin 4E caused
the split...I think it caused
a split. I don't think there's been any one single split, but many, many splits. And let's face it, the customer base has always been split. It's just that in the earlier history of RPG's there were less alternatives to choose from, and less vocabulary to put voice to the differences in playstyles. Inherently, I think RPG fans/customers are a very disparate lot - but I don't think that factor, in and of itself, spells doom for the hobby. If acknowledged and worked with (rather than ignoring it), it can be a strength to the industry.
IMO though, I do agree that there were splits long before 4E came along. 4E's split was simply that it mostly focused on one segment of the customer base, and in the vacuum of products at the time, 4E is what took the blame (among other reasons). Nothing against 4E. I think it's a good game. However, it is focused predominantly on only a portion of the playstyles espoused by the customer/fan base.
I think a game that has a common (as possible) base system, with modular systems that can be plugged on to make it the game each group wants, won't necessarily unify or heal the fan/customer base, but I think it definitely has the potential to heal and unify the market. But I say
potential because there's another significant factor I feel must be addressed: The GSL.
Overall, I don't think the GSL was entirely bad. Closing it up a bit as they did, would have avoided the diffusion of the system that occured under the OGL. But what killed the GSL wasn't it's lack of full openness (like the OGL), but it's poison pill clauses for publishers (revocation at any time, destruction of wharehouse product if WotC demands it, etc.). Take the poison pill clauses out, and for the most part I think it's good to go. Then it can again do what it was supposed to do, open the system up to 3pp to do what they do best (and what WotC struggles with): Adventures, Campaigns, and Supplements. And with the GSL, diffusion and fragmentation through derivative systems is probably much less likely.
