All in all, it’s not anyone’s place to play religious scholar with anyone else’s belief system or to demand hard proof either. Personally, I haven’t found a sect that really matches with my reading and understanding of scripture. So how do you validate that?
As for the second, no one said the “mark” has to be a literal mark. Many sects of many religions view large parts of scripture as allegorical or good general teachings for a coherent (if not imperfect, particularly based on modern secular norms) society.
No to be overly accusatory, but this all reminds me a bit of a young, agnostic me. The “well what about turning the other cheek” type of gotcha comment that ignores scripture that provides deeper context or contextual “contradiction.”
We live in a large and diverse world, and religion is ultimately a human system. We’re just never going to be able to nail it down to an instruction set as we may desire to do.