Let's be honest here. What part of Darwin's writings state the a beneficial mutation "guarantees the survival of the modified form"?

If not that, then how about any paper or book by an evolution biologist that states that a beneficial mutation guarantees the survival of the modified form?

You've done a vast amount of research so you must have come across a scientific paper that states this or implies this. I'm sure you wouldn't make such an sweeping statement unless you had read it in a journal, paper or book by a professional biologist. So, all you have to do is point me to it and I will read if for myself.