I'm hoping to understand what you are requestion.  It sounds like you would like a couple of things:

1. Grade a post directly in the discussion board
2. Flexible reply options in speedgrader
   1. Leave a private comment to a student
   2. Leave a public reply to all students

I don't know what your personal desk setup is or what your options are, but when I'm grading I typically have two monitors and on the left I have the actual discussion assignment, and on the right I have Speedgrader.  By the time I'm grading, the discussion has typically wrapped up, but if I wanted to leave a public comment then I wouldn't do that in Speedgrader.  That app is used solely for grading and providing feedback and the only time you would want other students to see your feedback to someone is if it is a group project/deliverable.  So in Speedgrader all replies are private and in the actual discussion all replies will be public.  The advantage I have when I pull up the discussion on the second monitor is that I can see the posts and replies in context.  That is a feature that Speedgrader doesn't currently have as it isolates each students' posts.