"Using non-local quantum entanglement to instantaneously transmit information indeed would be faster-than-light."
Yes, and entanglement can't do that as I tried to explain in my earlier post.
You do not transport information but quantum information with entanglement. They are as far removed from each other as Schroedinger's cat from any pet you've ever owned.
BTW tachyon are hypothetical faster than light particles. Of course you could signal into the past if such a resource existed and could be technologically controlled.
And once again, nature has no concept of "instantaneous" it's a human approximation to a constant frame without relativistic speeds. Whenever a physicist uses this word in connection with entanglement its an unscientific shortcut that has no real physical meaning.
I'd highly recommend you to draw up some Minkowski diagrams, until this clicks for you.
Understanding the nature of quantum information is a bit more tricky, because it all comes down to how quantum correlations will look like perfect random noise until you get the information from a corresponding measurement on entangled systems. The way this is experimentally tested is via Bell's inequality violations.
Bell was much later than Einstein. Bell essentially codified the latter's reservations and wanted to prove Einstein right, but nature did not cooperate.
BTW the only quantum spookiness that QM allows are Quantum Erasers.
And please note, that all these oddities are not hypothetical but demonstrated in experiments. It's these weird experimental results that have brow beaten physicists into accepting entanglement as a fundamentally non-local phenomenon, against the marked resistance of some of the best and brightest minds like Einstein and Bell.