So what good is it? If we assume the null hypothesis how do move to believing in the Bible? At best it seems arbitrary.
The question is important in that it highlights that there is no basis for belief.
There is no singular idea or bullet proof means to get theists to see that belief is vacuous. The best we can do is engage in dialog and discussion.
Skip the question in the OP. Get to the nitty-gritty and ask the YECs how they know the first part of Genesis is true if nobody could have been around to witness it. Similar basic question as the OP, but without the possible complications.
There is not, and cannot be, a case where the author of the first part of Genesis witnessed the events, let alone has evidence. Therefore, anybody who believes that first part of Genesis is true must accept that it is true on faith that God communicated that information. And if they believe that God communicated THAT information, then they can believe that God communicated ALL of the information in the whole Bible.
The argument in the Original Post is that before about 2000 years ago the Bible wasn’t written down, but people before then still believed in God without having a Bible to cite as the basis for their belief, so if there is a God those people before the Bible was written must have had evidence to support their belief, and if that evidence can’t be shown then God must not exist.
I respect that argument, but it is indirect and problematic. It is possible for some event to occur, and witness reports of that event to be carried along for some time, and all traces of evidence of the event to disappear, and then the information about the event being written down. The OP seems to imply that if there is no evidence, then the Bible is false. But as I just explained, it is plausible that there can be no evidence, but the event was true. So the argument in the OP does not necessary determine a true or false value.
But if you cut to the chase of the first part of Genesis, it is not possible that witness reports of that event eventually wrote down their information because it is not possible that there were and human witnesses. The author could not have written information based on evidence because the author could not have existed when God made light.
If you reduce the argument to this, there cannot be any evidence and there are no plausible explanations other than accepting the information on faith. Not evidence. Faith. Not plausible in any other way. No manipulations of dates. Alternative interpretations.
The first part of Genesis cannot be accepted as true account of a witness. The witness would not have existed (which is a similar, but round-about and not iron clad, argument the OP is making). And if there was no witness to the first part of Genesis, that person wrote it without evidence. So accepting it as true is a matter of faith. And if you accept that on faith, you can accept the whole Bible on faith, which makes the OP’s request for evidence of other things to support belief in the Bible irrelevant.