I just got confirmation that posting online is a planned feature that isn’t implemented yet, not something that is purposefully not allowed. Also, you are correct that ActivityPub is currently planned to only be one way.
Thank you for the detailed write-up! In general, we track the status of feature requests on GitHub, so the best way to ensure that these get looked at is to create an issue for each feature request in Issues · PretendoNetwork/juxtaposition-ui · GitHub. Probably link this post in the “Any other details” section for context.