As designed User State: Email invalid (bounced)

Yes. There is no permission on replying to a conversation you're already in. (This is used to ensure that admins can still interact with users that can't otherwise receive conversations.)

They won't receive any email from the conversation.
 
It's really just controlled by permissions (unregistered / unconfirmed). If you have any further questions, please post a new thread in the support forums.
 
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