@commie_sinbad This would make more sense as a separate tweet rather than a reply to this thread. :-)
It's outside of my expertise (and it is also outside of his). Hythem Sidky who can judge is not terribly convinced.
I can say a couple of things:

@commie_sinbad Shoemaker courts the idea that maybe parchment was stored for decades before it was used. This has always been a silly argument in my opinion, but I think by now it's indefensible. We'd have to assume that like 20 different manuscripts all had their parchment stored for decades.

@commie_sinbad If that were the case we'd see absolutely no correlation between radiocarbon dating and their palaeography. But we do. You have to come up with a lot of explanation for that. Shoemaker doesn't.

@commie_sinbad Shoemaker uses very shaky evidence to dismiss radiocarbon dating. He dismisses the Birmingham fragment as being pre-ʿAbd al-Malik because there are lines between Sūrahs. Who says that ʿAbd al-Malik introduced lines between Sūrah? Some random report from the medieval tradition.

@commie_sinbad The questions about offset and upper/lower hemisphere stuff is really outside my expertise, and I can't judge. It's a good thing to be aware of, people who know better do not seem overly concerned about it. I wait for a more informed researcher to address it.