Oh, you mentioned you don’t want to keep a backup of the entire drive. That is fine, but absolutely back it up before starting the install.
I would just boot a live Linux image and dd the entire device file onto some sort of storage. That way you can get a bit for bit copy of the drive that you can make it how it was before you touched it. When all is well, then you can ditch the backup. It wouldn’t be a bad idea to keep if the stuff is important. Storage devices do fail.
He is being too nice. He needs to get a lawyer and sue that shitty company for harassment and whatever else.
ETA: The US isn’t overly litigious. We are under litigious if anything.