By design there are several fields that are omitted for staged users in email templates (like the respond instructions):
I’m not entirely sure of the reasoning, jut stating that in code this is how it is, so you are currently fighting an up hill battle here because you are attempting to do something discourse is designed not to do.
Yes, you can edit the email templates, but you are pretty limited in what you can customize. Like I don’t think you can customize the email template to look one way for staged users and another way for non-staged users, but you can add a message with a link to the bottom of the notification email inviting them to visit your site instead of just interacting via email.