Yes, but again, we would not need as many crystals as dead ancients. They created him, not just summon him, so the aetherial requirement should be greatly reduced. And scholars for millennia have been studying the art of summoning, so the knowledge likely won't go anywhere.

But for the last point, even though we can undo tempering, if someone does summon a primal the likes of Zodi and knows how to bypass the amulets, tempering en masse may prove to be effective, even with a cure. If you have the ability to temper and the mind of a tactician, mass tempering may prove too powerful to simply just cure everyone. And if anyone comes after you, you can just temper them as well and be done since the blessing of light no longer exists after all those who now have it are gone. I never understood why the primals themselves didn't employ more mass tempering methods in the first place. The only thing I could think of would be, it would just make the story end lol.