Hey everyone!
Interested on the topic too. I don't have/use GrapheneOS, but am facing the exact same issue with LineageOS and I didn't find anyone else discussing the topic.
I even tried to see if a VM could help, I must say I didn't do anything like that before and had to teach myself a bunch of stuff, I tried both with containerized emulators and waydroid on arch, they work fine and very interesting stuff, but they all use some kind of OS (Lineage in the case of waydroid) that despite having the GApps installed, they get that piece of junk (Mobilepay) giving the same error (normally code 1, 10 and on other devices I saw more).
I took the time to write to the MobilePay support, well aware that they were not going to help. And I couldn't even get them to let "us" know what those error codes even mean. They told me that they don't have those code at hand, they can mean different things... morons.
So if anyone has some kind of constructive idea, inside a mobile OS or outside of it, very happy to try stuff and contribute!
I would actually love to find better means to exchange money and stuff than MobilePay, so even that could be interesting to discuss, even more if on the Open Source or FLOSS direction.
Cheers!