We already know the conclusion of this story: right-handed DNA is what exists today. The effort here is not to create a story blindly from known fundamentals i

We already know the conclusion of this story: right-handed DNA is what exists today. The effort here is not to create a story blindly from known fundamentals into wherever that story might lead. The effort is to fill the gap in story from known fundamentals to a known conclusion. We don't know if other circumstances could have lead to another conclusion, and that would indeed be an interesting question to answer. But we do know that the conclusion we are living in somehow favored right-handed DNA. If this backstory leads to two potential conclusions - either left-handed or right-handed DNA, and not both - and we are already experiencing one of those conclusions (right-handed DNA), then it is OK for that backstory to be ambiguous. We don't need to prove the impossibility of the left-handed result. We only need to illustrate the impossibility (or unlikelihood) of both conclusions existing together at the end of the story.