>> One theory that I thought was reasonable for why there's a monochiral world is that once the arbitrary choice is made (L or R) then that gets 'locked in' by all the machinery around that choice. As in, L 'won'.

This seems obviously true to me. Mixed doesn't work, so as molecules and systems of molecules started replicating one chirality won out. It's just chance and there's nothing magical about the chirality "chosen" by the process.