This really is an extraordinary and important book.
Interesting, but it doesn't begin to explain how some people escape the so-called contagion of mimesis.(That the same part of the brain lights up in both observer and observed, while well documented, does not actually establish whether desire is affected. After all, when I watch someone drink, I do know that I'm not the one who is drinking. Maybe it's proof of quantum entanglement...or telepathy...or??)
I don't know that anyone entirely escapes it. Girard's ideas are both complex and nuanced.
I believe you're right that no one entirely escapes it, which probably means it must be a survival instinct/mechanism. But this begs the question of why it affects some of us more than others.