I have never used Rohacell, but anything that has porous surface needs to be filled in this manner. I built a 22.5lbs 33% Extra 300SX out of Divinycell last summer, so I have gained a lot of experiance with this material.

After rereading your process, you should be able to use just the 2oz glass and your carbon reinforcements and balsa, you could use even lighter considering I am using 1.4oz glass on my 35% Extra wings.

Make sure you have the glass oriented the same (45°) on your laminate, or on a hot sunny day you will learn what thermal expansion is the hard way. You can add layers all you want, but put them in the same orientation.

Like this:

2oz@45° / 2oz@90° / balsa / 2oz@90° / 2oz@45°

These layers will heat and cool the same and will not twist the piece as the day goes on.

Darrin C