This may seem a bit farfetched, but I may have a solution, or at least the start of one. I noticed Hallo
Keep the composer as before, but rather than having a seperate preview zone, it would just appear as if it were an actual post, then would be seen by others once posted.
Made you a mockup. A further advance to this would be make the post editable from the actual post, without needing a composer. But I'm sure there's some limitations with all of this.