My initial hypothesis is that there's something present in the early stages of life that has a higher energy state making it unsustainable for use in a certain conformation and so it was nearly immediately selected out.

E.g. a ring structure whose substituents are affected by steric hindrance in the left-handed scheme.

And the path of least resistance was just to adapt and build around it. Once that precedence was set everything became as such. I expect in the earliest stages of life this would have been an immense factor as metabolism was not nearly as sophisticated as we know it today.

And this selective process may have ocurred well before anything we have observed/modeled, and may well be erased. Which is to say I agree, but with the caveat that it was a substrate-dependent mechanism which selected the downstream components rather than random chance.

Seems quite possible, but the difficulty would be why one enantiomer is less favourable than the other.

Totally agree that it is hard to test these things experimentally, or through historical analysis of structural remnants. I understand there have been efforts to model at a system level these ancient metabolic networks but ... then how do you experimentally validate these models?