"My understanding is that it's possible (albeit staggeringly unlikely) for any possible structure, including living beings, to spontaneously form via quantum fluctuation, and given infinite time and space every possible structure will spontaneously generate."

That was exactly my perception, specially given infinite time, not to mention space, so here is Infinite squared, or Infinite x Infinite, because there are two Infinite fields of possibilities.

Off course I didn't have any Idea about the existence of Quantum Fluctuation, just was my intuitive perception. And thank you for teaching me about It's existence.

"If the many-worlds theory is true..."

This is already too complex for my Simple Mortal Mind to understand, at this moment of my lifetime.