Just because something isn't "real" for one person as it may be for another, doesn't make the delusions of that person valid in any way shape or form.
As for disproving God, there is nothing to disprove, the burden of proof is with the believers and the claim that "you can't disprove it, thus it's an option" isn't a very good one. As for the Big Bang question, just because some may see it as an act of God, it does not make it so, please understand this. I could choose to see it as the results of a farting unicorn but that does not make it a form of reality unless I have evidence to back it up.
Your 4 witness accident thing is flawed. It depends on how much they vary, if for example one witness saw one side of the car while another saw the other side, then yes, they will be both talking about the same accident albeit different accounts of it. But it's still the same accident and if anybody was standing in the same position as either of those guys then they would be able to witness that same thing... because it's there.
Now, if one of those guys says that they saw a fart propelled unicorn fly out of the windshield and nobody else could see it, sure it may be reality for that person because they might have deluded themselves into believing it (shock, psychological problems and various other things could have taken place), but if there's no evidence suggesting it's real nor is it observable in any sense by others, then nein.