So apparently her protection spell strong enough to deflect undeflectable killing curses has the basis in her throwing her life in front of Harry's even though Voldemort would let her survive.

This is a lie. This is Dumbledore whitewashing the history of what happened. Her protection is vaguely referred to as a charm or some other such mundane school or technique of magic... but it's not.

Lily, in her last moments of life, cast a Blood Magic spell of which all are forbidden to the point where students aren't aware they even exist.

Specifically, she offered Voldemort a contract, writ in blood and Voldemort signed it. Then he immediately reneged on the terms.

Recall her last words,“Not Harry, please no, take me, kill me instead–”. Recall that she is wandless. Recall that everyone in the know refers to this as old magic.

And recall of course, where the power and protection linger, what Voldemort forceably takes from him, and to what harry must return to every summer. His blood.

This would mean that if Voldemort killed Harry first, and then killed Lily there would be no unexpected adverse affect to Voldemort (he was splitting his soul again).

I could be wrong, but that's my read.