I think that comfortboard is fine. I'm pretty sure I've seen others use it. The best way to combat any shortcomings among the various treatment options is just to install more of it.
While I'm not sure I completely understand all of your post, setting up a Reflection Free Zone is more than likely covered in the stickies or for sure in my listening room pdf at hexspa.com/listening-room-ebook Long story short, you put panels half way between you and your speakers on your side walls and ceiling.
More than likely, you don't need to cover the whole room with 4" insulation it's just that it would cover your bases. Like I said, RFZ+checkerboard 4" everywhere else plus some substantial fluffy in your four vertical corners and you should be 90%+ there. This is the best advice I can give without actually being there and doing it myself. Thanks.