But isn't that something that's allowed regardless of a license? Plenty of cell phones accessories by third parties explicitly claim compatibility with famous brands.
I don't know about the rest, but I'm fairly confident that we would have had some kind of OSR. It was born out the retro-clone games and those were in "reaction" to 3rd Edition and the abandonment of TSR-era D&D rules by WotC.
How can you give inspiration once per turn? Heroic Warrior triggers only if you don't currently have inspiration. OTOH, you can give out inspiration only if you gain it while already having it. Unless I misread the rules.
So far, I've been using the years, so the first option for me. It's not a big deal to me, though. If something else gains large traction, I will be likely to switch.
Is it possible to publish a product using both material released under the OGL and CC?
Edit: Just a general question, I'm not asking for legal advice. :)