ciao-hound Does each user account have a Private Space, or is it one per device?
Currently only the owner profile has a private space, but the GrapheneOS team has mentioned the possibility of supporting one for every profile. Not sure where that would fall priority-wise, though. Seems like there is a lot going on right now.
ciao-hound Can apps in the PS use a VPN in that space, while still running a VPN in the main account?
Yes. The owner VPN is not used by apps in the private space. They need their own. Just like other profiles.
ciao-hound Also, is a Google account necessary to set it up?
Not at all.
ciao-hound What is risk/benefit of using one?
I'm not sure there is really a risk. The downsides are inconvenience compared to having all apps in one profile. And I think some apps may have issues running in a private space.
One risk could be accidentally sharing data between apps that you intended to keep separate.
The benefit is the additional isolation of apps, similar to having a separate profile, but with the ability to manually share data if desired, and without the invasive privileges needed for a work profile. Also seamless switching between apps in owner and private space.