I propose this definition of "Myth":
a traditional or legendary story.
For the record, I acquired this by paring all the uncertain verbiage ("usually", "with or without", "especially") out of this definition:
a traditional or legendary story, usually concerning some being or hero or event, with or without a determinable basis of fact or a natural explanation, especially one that is concerned with deities or demigods and explains some practice, rite, or phenomenon of nature.
So that's not much more helpful that just saying "Mythology".
While I understand the allure, I think coming up with a canonical definition by which all other topicality questions can be judged is just infeasible, and liable to cause confusion rather than alleviate it.
I think the right thing to do is deal with topicality issues as they come up.
Most definitions we come up with are going to try to narrow it down based on what the purpose of the story is, but that's the most useless possible guideline, because it's based on interpretation instead of content. Interpretations are unclear and different people come to different conclusions.
Or we try to base it on societal/cultural impact, but that's not useful either. Orwell's "1984" and Twain's "Huckleberry Finn" are two fictional stories that have had a deeply profound impact on modern society, but you likely wouldn't regard them as mythology.
Or we try to establish a type of group that must believe the truth of the story. But a religion is no good, neither is a nation. Not enough latitude. There we might start talking about cultures, but there we're back to too vague to be useful. Sure, Greece has a culture. But so does Coca Cola and a community book club. (If you ask me, the best way to make the term "myth" more confusing, is to bring the word "culture" into it)