Ok, we can get through this. I have made several of these. It has been awhile but it is not too bad to do. You will have a great bag when you are done.
I think you may be over thinking this which is easy to do. You are going to end up with your zipper sandwiched in between the two larger bottom pieces and then on top you will have the zipper sandwiched between the two smaller pieces. You will end up with a pocket that are finished on the front and back. I think you may be thinking that you need to be able to access the area between these two pieces. Not so. You will take this "finished" zipper pocket and sew it on top of the bag front. Baste as directed. The bag front actually becomes the back of the pocket. When this is basted onto the bag front, opening the zipper will get you to the area between the bag front and the two layered pocket. I am really awful at writing directions so if there is a way I can help you with pictures or verbally, just let me know. It really is totally do-able.