Why should the Gospel of Mark be accepted as any sort of specific historical evidence for any of its claims?
As a matter of historical methodology, by default I suspend judgment regarding the truth or falsiy of an empirical claim in any historical document, religious or secular. I only accept a historical conclusion if there is some good reason or evidence for doing so. Regarding the Gospel of Mark, I evaluate each of its empirical claims on a case-by-case basis. If there is an inductively correct argument for one of its claims, then it is logical to regard such a claim as probably true.