Indeed, if an Enochian fragment is found and dated to 500 B.C. or earlier - I'll give you a heads up!
I have not yet looked for information on physical dating (e.g. carbon dating) on the parchments or the containers, but that's my next step.
I realize that the contextual points hold no sway to your view. Nevertheless, to me, it is significant that the manuscript from which the Qumran copies were made does not speak of Moses, the law or related Jewish traditions as being in existence, though it originated in Judea and that would be “second nature” to the residents.
As one of your sources said, it's not all that remarkable. If you were going to write something purportedly by a pre-Flood author, you would of course omit things that happened later. It's easy to leave things out. More significant are those things that are included in Enoch. And looking at them, there is nothing remarkably prophetic. Unless, as you say, a fragment can be decisively dated to a time that would make such writings miraculous indeed. But so far, I see no need to leap to unwarranted conclusions. I'm always open-minded to solid evidence, however. Keep me advised if something turns up.