We plan to install Englert's standing seam metal roof which comes in 16" width with a 1" high seam. It is a snap lock design for easier installation. We will probably apply ice shield over the entire roof surface of the dormers, as they are not that large, and plan to apply it, as you recommend, to the lower 36" of the main roof.
The sheathing is 1/2" plywood. The issue of removing the sheet rock stems from the fact that the new windows are much larger than the old ones and will take up much of the wall space. There won't be much old stuff left on the wall, anyway. That is why we thought we should replace the old insulation while we were in there, but I also wanted to include a thermal break. I like your idea of cellulose, but that gets back to the question of rigid insulation. If the rigid insulation acts as a moisture barrier, should it be applied to the inside, perhaps as the cellulose is being blown in? I know applying it to the outside would be easier, but it seems like we would be trapping moisture.
The windows will be high performance Marvins with simulated divided light.