The same version 18.09.1 is also available for Fairphone Open. However, we do not call it a release candidate, because it is missing features unique to Fairphone Open. Most and foremost, it is missing superuser which we still need to get to work nicely.
Yes, that is totally possible. It will not be necessary to wipe data to go from Android 7 beta to Android 7 stable. In the best case, this release candidate will be the official update in a few weeks.
Opting out of the beta is always possible by disabling beta mode in the updater.
Good point. I added a disclaimer to the post above linking to the known issues.
No, we use the same baseband version we used on Android 6.