And what is the effect on the reliance of third parties upon this wording to enter into independent agreements with licensees covering IP not under WoTC control? Must they cease using the OGL version 1.0a? Or do these sublicenses survive termination of the parent license?
What you're describing don't sound to me like sub-licences.
But anyway, the use of the OGL by parties to licence IP among one another that is independent of WotC's IP claims seems to me to be unaffected by any decision WotC takes,
except for the points I made in post 364 in reply to you.
First, PF1 was built on the SRD from WotC. If that is no longer licensed (because OGL v1.0a that granted them license is gone), do they still have the right to license the formerly-licensed open content?
You wording is a bit confusing. The issue is not whether "OGL v 1.0a which granted the licence is gone". Its whether or not Paizo continues to enjoy a licence in virtue of its entering into a licensing agreement with WtoC. And this has been discussed upthread.
@bmcdaniel said that the better view is that WotC can't unilaterally end its agreement with Paizo, but that if it now revokes the offer to enter into new licences, it's unclear how that would effect Paizo's ability to sub-license into the future.
One view, which I think is quite plausible as a matter of contractual construction, is that insofar as Paizo retains its rights under the agreement, one of the rights that it retains is to continue to license OGC to other parties. But given that an expert has said this matter is unclear, the view that I think is plausible can't be regarded as anything close to certain!
Second, if OGL v1.0a is revoked (WotC has the copyright on the license itself and can revoke our right to copy it, rendering it impossible for us to comply with its terms that we include it in derived works), does Paizo even have a license they can use to license their open content, if they still can do so because they no longer are licensed themselves?
Paizo's right to reproduce the text of the OGL is conveyed to them by the terms of their licensing agreement with WotC (which are the terms set out in the text of the OGL). So if it retains its rights under the agreement, it retains its right to reproduce the OGL text as best I can see.
Whether WotC really has the power to render it impossible for others to copy the text of the OGL I don't know - see my post 364 upthread.
So effectively, if you do not opt-in to the OGL v1.1, you can continue to exercise the rights granted under 1.0a?
Perhaps. I think this is a plausible view. I suspect that it is the best view. But as I've said, in this post and my earlier reply to you, I don't think it is certain.
Particularly because different categories of rights enjoyed under v 1.0a might be affected differently by decisions made by WotC - eg whether one can continue to publish existing licensed OGC involves a different right conferred by WotC under the agreement (namely, the right to reproduce their copyrighted work), compared to whether one can create new sub-licences for the use of WotC's work (which is about a power of agency or similar conferred by the OGL v 1.0/1.0a on existing licensees). A decision by WotC to retract its standing offer to licence the 5e SRD under the OGL v 1.0a could effect these two rights held by existing licensees differently.