Yes, Discourse users can still reply to 2a. We have this working! So when new Discourse messages are sent from community members, it automatically is added to the Front thread.
This screenshot may help illustrate better:
As you can see, I replied via Front “Just a test message from Front” and because that is posted to the Discourse, the Discourse API picks up the fact that it’s a new message and therefore sends a fresh message onto the thread.
Your reply has got me thinking 
Maybe creating some sort of function that looks for an id from Front to then exclude it in the Discourse SEND MESSAGE API… I’ll give that a go. Appreciate the help! Thank you 