For example, we know that at least 12% of English is from Greek,( 35% according to some linguists), and we dont have any evidence for most of that, but does that mean it is not a fact?
This makes absolutely no sense to me. Whether or not the above %'s are true, if we have no evidence for it, it is NOT a "fact", it's merely a speculation.
The pronunciations of Harry and Henry were IDENTICAL in medieval England; that's the point that experts on the history of names are making when they show the connection between the two. The modern English pronunciation of "Henry" is newer than the pronunciation as "Harry", based on Norman French. It is of no consequence how old "Harrys" is in Greek in deciding whether or not the modern English name Harry is connected with it.