Hi Frank,
Yes, you can build the wall, but I would leave it acoustically transparent (relative term) and not insulate the wall itself. Do it behind the speakers. However, this all depends on how much depth you have allowed for your speakers to be placed. In addition, using standard pink insulation is nowhere near dense enough to provide any adequate sound absorption in the lower mid-range frequencies (the middle of the audio spectrum is 500 Hz). Place it on the wall behind the speakers, but this leads to the next problem as you need high efficiency absorption that can handle speakers being close to it due to SBIR...again this leads back to how much room you have left for your speakers between the AT wall and your back wall. Regarding other design issues, I would look at possibly trying to make the room as rectangular as possible and leave the doorway you have depicted near the front right side out of the equation...or at least place a door there. This looks to be right around your first reflection point which is highly important. You need to place treatment right around there, but I would not go with strict absorption at this location as you will literally collapse the front sound stage and make the room sound as big as it really is. I aim for trying to expand beyond the walls. Also, leave your speakers enough room to play with them. Strictly adhering to a specific spot will not yield great sonic benefits for you unless specifically designed to do so. Hope this helped!