In the BIOS set the "Init Display First" to PEG for PCIe. Also Right after that in the manual under "Internal Graphics" set that to Disabled
For booting off of the USB, F12 and choose the UEFI USB option. At the Clover boot menu, choose to boot your system drive and not the installer. I just did this with a UniBeast Drive to verify, so I know its possible.
In the picture I posted of the nvidia device manager with the Web Drivers selected, is that what it looked like for you, when you said they were already installed? The only way for them to get there is to DL and install them manually. That is what I wanted you to do.