What is the best guide for the intelligent layman concerning the topics listed in the title?

Two questions I’m particularly interested in hearing about:

–Why do we know that entanglement is due to non-locality and not due to hidden variables? Because of experimental results interpreted under Bell’s Theorem, of course, but my question is, how was Bell’s Theorem itself demonstrated to be true? (In summary, for the layman, if possible.)

–Measurement of an entangled particle results in a fixing of its partner “instantaneously.” What instant is that? What does “instantaneous” mean when, acc. to Relativity, there is no such thing as absolute simultanaity?

Thanks for any help,

-Kris