moddel If a new pixel phone was purchased today, sold from an authorized retailer as an unlocked phone, is it possible to toggle the OEM unlock setting without internet access and install GrapheneOS directly?
It depends!
If the device variant is never sold to carriers that lock phones then the OEM-unlock toggle may well "just work". I believe that in some countries locking by carriers is illegal and thus device variants targeted at those countries may not require Internet access for OEM unlocking.
moddel Or is it a hard requirement that the phone requires an internet connection and communication with a Google server in order for OEM Unlock to be activated.
For some devices it is a hard requirement. I suspect it's a hard requirement for all Pixel devices intended to be sold in the U.S., because some major U.S. carriers do network locking, and Verizon Wireless completely forbids OEM unlocking. I suspect that from Google's perspective it's a big win to be able to manufacture U.S.-bound Pixels all containing exactly the same software, and then, as carriers purchase phones, to retroactively tag those phones as administered by those carriers.