They said that, but I couldn't find the fix in their github repository. Does anyone know the commit that fixes this bug? It would help shed light on the technical details of the issue.
GrapheneOS is spread across many many repos. They are listed on the website: https://grapheneos.org/source#grapheneos. It's impossible to put an entire operating system stack into one single repository.
I used GitHub's search across the entire Graphene org, but couldn't find any references to the issue. Doesn't mean its not there, I just couldn't find it. If the technical details of the issue were public, it would help people know whether or not their data is recoverable if they are already stuck with a "pixel is starting..." phone that doesn't boot.
I have no idea. For your privacy and your freedom it's best not to use these corporate surveillance systems anyway. Use cash whenever you can or if necessary a credit card with contactless payment that you keep in an RFID-blocking case when you don't use it. There might also be prepaid credit cards that support this, I don't know as these are only common in North America and I don't live there.
By looking at this page on their website. But I can also just tell you the answer: No, unfortunately it's not supported. There are quite a few reasons for this. The Galaxy S7 is old and abandoned by Samsung, meaning it doesn't receive kernel and firmware updates anymore. Even if it ran the newest release of Android, it would still lack important patches and potentially be insecure. There's another reason why the GrapheneOS team decided to only support modern Google Pixel phones: Hardware security. Google pixel phones are equipped with what is known as a 'secure element' with their Titan M chips. These are tamper-resistant and can thus safely be used to store cryptographic keys and other secrets that are e.g. used for full-device encryption.
Edit: I did some research and apparently it's a japanese mobile payment system that utilizes NFC. I found this on the GrapheneOS issue tracker, it seems to have been broken, but apparently it's fixed now. Unfortunately I can't try it out as I'm not from Japan. I also found this in the patch notes of a GrapheneOS version that was released in March of this year:
include PixelNfc app on all supported Pixels to enable support for FeliCa on Japanese Pixel models