Bootcamp software won't install directly on Windows 10, doesn't support Win10 on my machine (trust me, I tried) - that means I'd have no boot into Mac option, requiring a UEFI compatible GPU to access the boot menu upon reboot, hence the install Win 7, Bootcamp, Win 10 procedure.
I really do make things hard for myself.
As for the serial key thing, I guess I hadn't run the latest updates at that stage.