whether make use of nightly versions is welcome
Yes, it is, especially if one reports bugs back to us including the nightly version so that we know it’s not a bug on the stable releases 
whether they are possible to use if having not connection and account on google services?
This should work too. We don’t use any gplay services, so it’s only signed with the gplay certificate (I think). I used the nightly version already on some phones without any google stuf finstalled.
If understanding right in regard of new installation of app, it’s required to save a backup first and new app would fetch if automaticaly from the download folder, right?
If you switch from the fdroid version to a nightly or gplay version, you have to export and import the backup as the appid changes. If the appid stays the same, everything is fine and no need for this. But if you play with nightly versions, it’s possible that coming back to an older/stable version is not possible. Sometimes the database is getting upgraded if the core version changes, but downgrading is not possible.