Regarding this use case:
What should happen after you’ve sent a new reply in with your non-primary email address? Should that email also be subscribed to the thread now? As far as I can tell, that would get messy.
My suggestion would be that whenever you send in a new post using your non-primary email, you get a confirmation email back from Discourse saying:
Your post, sent from jane-social@example.com has been received and published. Please note however that you will only receive updates on your primary email, which is listed as jane@example.com.