I've only just started reading through your tutorial. It's very impressive.
I did notice a small error. Under section 2, "Custom User Entries", you have:
It should be:
Any created file must be made executable. This can be done as root by running "sudo chmod 644 /etc/grub.d/filename
sudo chmod 755 /etc/grub.d/filename
The latter is probably easier to understand for those who are not familiar with the numeric mode and it's consistent with the other uses of the chmod command in your tutorial (for example, where you have turned off the executable permissions with "-x").
sudo chmod +x /etc/grub.d/filename