Seems a bit convoluted to me, I simply go
The simplest way to upgrade is to enable trusty-proposed, upgrade ONLY packages whose names start with "grub" (e.g. use "apt-get dist-upgrade" to show the full list, say no to the upgrade, and then pass all the relevant package names to "apt-get install"), and then (very important!) disable trusty-proposed again. Provided that there were no errors in this process, you should be safe to reboot. If there were errors, you should be able to downgrade back to 2.00-22 (or 1.27+2.00-22 in the case of grub-efi-amd64-signed).
If it checks out then remove the -s & do for real
sudo apt-get -s install --only-upgrade grub*