A question: Is WotC a Contributor?
First, an analogy.
Wherever you live, there are probably laws about bicycles - for instance, about whether or not they can be ridden on footpaths, about whether or not they must have lights and a bell, about when and how cyclists must signal turns and changes of lane, etc.
Those laws will talk about "cyclists" and "bicycles" and the like.
So then you could ask, Am I (pemerton) a cyclist? Is my bike (that I ride around on most days) a bicycle?
And the answer is - I am the rights sort of entity to be a cyclist (ie a human being who rides a bike from time to time) and my bike is the right sort of entity to be a bicycle. But I am not governed by the law in your jurisdiction about cyclists and bicycles, as the law in your jurisdiction does not purport to govern what happens in Melbourne.
It's conceivable, of course, that a legislature that exercises authority in Melbourne might borrow the text of your jurisdiction's law, and enact a law in exactly the same terms in Melbourne. But that still wouldn't make me a cyclist in your jurisdiction. I would be a cyclist in a different jurisdiction that happens to be governed by laws that are expressed in the same terms as the laws in your jurisdiction.
Second, to turn to your question:
WotC is the right sort of entity to be a Contributor - it is a company capable of owning and licensing copyrights and trademarks.
When WotC enters into a licence agreement in the terms of the OGL, it becomes a Contributor within the context of that agreement. And as WotC's licensees sub-license down the chain of agreements, WotC becomes a Contributor in the context of more and more agreements .But it would be an error to suppose that this makes WotC a Contributor for the purposes of other contractual relationships to which WotC is not a party, simply because those relationships happen to be expressed using the same wording as WotC has used for its licence agreement.
I’ve been taking been taking this as a given, but I can’t find a clear statement anywhere that they have “Contributed” OGC. They have made the SRD available as OGC but not by “contributing“ it through The OGL. They did so under the original “top level” OGL.
That seems to make OGC of two types: OGC as made available by WotC through the OGL, and additional OGC as contributed through the OGL.
This is not correct. When WotC licenses its OGC, on the terms set out in the OGL v 1.0a, it enters into an agreement, which becomes binding in virtue of the exchange of consideration is set out in section 4. WotC - the Contributor (ie the copyright and/or trademark owner who has contributed OGC- see section 1) - licenses its OGC (ie the OGC it has set out in its SRD) to the other contracting party, the licensee. That is the consideration that flows from WotC to the licensee.
It might be worth adding that if WotC were not a Contributor within the meaning of section 4, then it would have licensed nothing and all uses of WotC's copyrighted text would be an infringement of WotC's copyrights. That's a sufficiently absurd conclusion that it should prompt you to re-read the contract in light of my previous paragraph.
Then the original OGC never gets swept up in clauses 2-4. Licenses which are granted by the OGL are only to additional OGC as contributed through the OGL.
That is to say, there is the original OGC, which is only available through the OGL, and then there is Contributed OGC, which is available through sub-licenses per sections 2-4.
To say, I don’t prefer reaching this interpretation. However, it does seem to most cleanly fit.
It's an absurd reading for the reasons I've just given. The whole point of OGC publishing its SRD with a notice of OGC and an offer to license that OGC in terms of the OGL, as section 2 sets out ("This License applies to any Open Game Content that contains a notice indicating that the Open Game Content may only be Used under and in terms of this License"), is to enable parties to accept the offer as section 3 sets out ("By Using the Open Game Content You indicate Your acceptance of the terms of this License"), thus triggering the exchange of consideration as section 4 sets out ("In consideration for agreeing to use this License, the Contributors [in the present context, WotC] grant You a perpetual, worldwide, royalty-free, nonexclusive license with the exact terms of this License to Use, the Open Game Content").