Will answer all of your questions/replies here:

You can switch between Stock and GrapheneOS without issues but you will lose data every time you do this, it's up to you to back up your data. The install page shows how to start going back to Stock.

Magic Eraser is a feature from the photo editor rather than the camera. You can use Google Photos on GrapheneOS but it has a hard dependency on GSF so you need to install the sandboxed Google Play apps. A lot of their editing also requires internet access and is done via cloud.

All apps on Android are sandboxed (that's why you can control permissions of apps) but GrapheneOS improves that sandbox (Storage Scopes, Contact Scopes, Network permission toggle) and runs a compatibility layer to allow Google Play services or apps to run in an unprivileged, sandboxed manner. LineageOS doesn't make any improvements, and many changes they make from AOSP have flaws, it is not hardened.