Allows network admins to send a manually created notification email to all registered users based on user role.
No. This plugin only works (and would only make sense) in a multisite install.
The plugin now has its own menu in the network admin dashboard.
Potentially, but as I stated in the description, I would imagine it would be a pretty inefficient way to spam users.
Possibly, but I haven't tested it myself. I don't see why it wouldn't work, but I can't vouch for it personally.