1) Normally, when selecting an option in Windows, there is a confirmation step which doesn't seem to occur here. When I press the "7" key or the "F7" key on the Troubleshoot/Advanced Options/Startup Settings page, the computer goes directly to reboot without confirming my selection.
2) When returning to the Troubleshoot/Advanced Options/Startup Settings page, the seventh option is always to "disable" the certificates. It would seem that if the certificates have indeed been disabled, the option would change to "enable" on the next visit.
It certainly seems like there may be a bug at the point where you try to disable sig verification. Instead of making the change, something goes wrong and causes your system to reboot before making the change. Microsoft seems to take signature verification for drivers very seriously given how hard it is to disable, so this might not be an easy fix.
I wish I could offer more suggestions but I'm just not that much of a Windows expert. I would suggest you find a forum where you can speak to someone who knows Windows inside out, especially low-level operations like drivers. I'm sure there is some way around this, but it would probably take someone who knows the ins and outs of how and where drivers are installed to do it manually. One of the Stack Exchange networks (
Super User seems like it would be most appropriate, though
Stack Overflow might work as well) might yield you a solution. Alternatively, you might try
Microsoft's own answers site.
Have you tried to contact HP? Putting some pressure on them might help as well. If HP released correct drivers that would be the most correct and easiest solution.