>About the OEM, are you working on having devices ship with GrapheneOS, or devices be GOS compatible (i.e. same as the Pixels)?

As far as I'm aware as an outsider, the aim is a device that is compatible with GrapheneOS like the Pixels, yes.

>If you're thinking of devices shipping with it, would this fix the issue of Play Integrity/SafetyNet failing?

I think to pass this you need to be 'blessed by Google' which means being certified Android by their standards. GrapheneOS have mentioned that their CTS/CDD Android certification process holds back some of the privacy/security features (think things like new Sensors and Internet permissions etc.) implemented so they cannot can't target it.