Er, yes, they are. They would certainly be admissible in a court of law seeking to establish identity.
Then you postulate fraud on the part of local government officials tasked with recording such information. Do you are any evidence to support this accusation of fraud? No? Let's move on then.
Whether personal papers contain 'truth' or not is irrelevant to determining whether they were written by the person in question or not. Also, those personal après were in the possession of members of Darwin's family who seem to have believed that ther husband/father wrote them and, according to their individual experiences, believed that he existed, cared fr and lived them. Do you have any evidence that any of these observations are false, that his family manufactured his existence and wrote his papers? No? Let's move on then.
Well, they are consilient with an array of other evidence that indicates they are. Do you have any evidence that that indicates otherwise? No? Let's move on then.
Well, lots of them have been identified by role who ostensibly knew him or met him as being of Darwin. Do you have any evidence that these many people were mistaken or liars? No? Let's move on then.
Although newspaper editors and journalists may indeed be mistaken in things they write or even just make stuff up from time-to-time, your postulate requires systematic, prolonged and widespread fraud. Do you have any evidence of such massive fraud? No? Let's move on then.
In which case you postulate extensive fraud on the part, not just of scientists and teachers, but of clergymen as well. Do you have evidence of such fraud? No? Let's move on then.
Absent evidence of the widespread fraud required to invalidate this evidence for Darwin's existence, the parsimonious expanation is that he did, indeed, exist. Should you wish to contest this understanding, you would need to present he extraordinary evidence to support such an extraordinary claim.
Well, ask yourself whether or not you find similar 'evidence' for the existence of Odin, Zeus or Lord Brahma persuasive and why this is or isn't the case and you will have your answer.