> Is it possible for users to unsubscribe from all Topics (my default setup) in one action?
Users can see their subscriptions in the bbPress profile page. They can unsubscribe from there, but not from all with a single click.
> Is it possible for users to select only topics they want to include in the digest?
The digest add-on captures bbpnns notifications and saves them for when it’s time to send out the digest email. You can configure bbpnns to use the bbpress subscriptions, so users will only receive notifications for forums/topics to which they are subscribed. That way, the Digests emails will only include topics/replies for users’ subscriptions.
> Or Digest include all topics for all users once you click on UPDATE ALL USERS.
UPDATE ALL USERS only assigns the settings in that screen – frequency, whether digests are enabled or not, etc.
> I’m not sure if this plugin overrides the notification bbpress options or not.
The digests add-on does not. It is an add-on to bbpnns, which does override bbPress notification options. That is the plugin that you will need to configure.