The nature of JN patterns (or any PP pattern for that matter) you may not always have the option to press your seam allowance to the dark. I would also have concerns with PP with a fabric that has interfacing. If the muslin if very guazy, I would think you could double it but then you may have an issue with too much bulk in the seam allowances, especially with many of JN star patterns, you will have areas of bulk in star centers. I would be very hesitant to use a gauzy muslin in this particular case. Do you have a quilting buddy or friend who could maybe give you a more substantial white until you are in better financial shape? I know you can pick up Kona solid or Moda Bella white on line for around $5.50 per yard.
Regarding your dark batting, no. It will shadow behind the white and cause it to look gray and dingy. You will need to use white batting for this.