fade
I think you have a good plan. In the temps that you mention - I would not wear the inners with spandex because you would be uncomfortable sleeping in them, however I would wear the ski pants. I mostly ski in loose mid-weight long underwear bottoms with full zip packlite pants over them with the tops of the side zips open for about a foot, this lets the pants breath and prevents over heating but allows easy adjustment by simply zipping up more. Your legs of course are connected to the rest of your body and by not over insulating the top, the bottom will overheat less. So I wear mid-weight long underwear top with fleece jacket. I carry a packlite jacket and if too hot I change the fleece jacket for the packlite over long underwear, of if too cold I put the packlite jacket on over the fleece. I have a fleece jacket and a packlite jacket with matching pit zips so I can easily unzip both to vent under my arms. Its just my way - warm breathable insulation under - water tight easily adjustable shells over.

Since you lose most of your body heat through your head, especially if you have male pattern baldness, a warm hat with ear covering and a baseball cap will also help regulate your body temperature.

In spring skiing I would probably not wear the ski pants but just the long underwear bottoms with high topped gaiters, but I would have the pants along in case it got cold. I also carry a light down jacket in my ski pack - something like a TNF nuptse that can be put on under the packlite jacket and over the fleece jacket.

If you are a very aerobic skiier disregard what I said. One rule to skiing with a pack and camping - do not sweat, take it easy and vent before you get damp, you can always warm up more.
Jim YMMV