It's the other way around. Google specifically blocks Goodlgle Pay being used on GrapheneOS. I believe there's another contactless payment app called Curve or something that works on GrapheneOS. Requires Google Play Services installed though.

Probably not if it depends on Google Pain Services?

No.

But third party payment apps might work. I'm using one for contactless payments and it works just fine.

No but nfc is present and nfc contactless pay works if your banking app has it native in their app. Curve pay is a good workaround.

Yup, it uses a sandboxed version that you can control.

Edit: Fuck me...it's early and my dyslexic ass read "play" not pay.