Alright, so

• A - Origin community
• B - Other community
• C - following community of A & B

User posts to A, a "as:announce" on C is generated. A user replies to the post on C. Will user A see the reply? Will someone looking at the post on A see the activity on C?

Yes. When the reply is posted to C, it is sent to A. A then sends as:Announce to C, as well as any other communities that follow it.

B seems to be irrelevant here.