On the first question…
What's most widely agreed upon—based on the evidence available—is that the Guidestones are meant to instruct the dazed survivors of some impending apocalypse as they attempt to reconstitute civilization.
…
What in the world would it be for? Fendley asked. Christian explained that the structure he had in mind would serve as a compass, calendar, and clock. It would also need to be engraved with a set of guides written in eight of the world's major languages. And it had to be capable of withstanding the most catastrophic events, so that the shattered remnants of humanity would be able to use those guides to reestablish a better civilization than the one that was about to destroy itself.
Moar…
https://www.wired.com/2009/04/ff-guidestones/