Patty, I don't think that you will get an answer from me about down pants.
I've led a lot of beginner snow camping classes where half used cross country skis and half used snowshoes. We would typically travel in only about 2.5 miles to camp, basically because I didn't want beginners to be completely wasted when they need to set up camp.
So, during the travel, they would be wearing really thin stuff, maybe only thin Lycra. As camp got set up, they would often be donning wool trousers over the Lycra. Then only if it got super cold would they put on rain pants over the wool. I don't believe that anybody ever wore down pants.
When we do long ski tours, like six days, nobody would ever take down pants.
Now, if you have a sleeping bag and you need to stretch its temperature rating, there are a couple of practical ways to do it. If you need the stretched rating for only one or two trips, then they way is with an additional sleeping bag used as an overbag. That can be just about any old sleeping bag that you borrow from a friend. Besides, you have friends who would loan you one, and they wouldn't be using it in winter, would they? The other way is to put more insulation inside your sleeping bag. I've seen all sorts of things used there including wool blankets, down jackets, wool sweaters, etc. Some people have used multiple hot water bottles inside the bag.
Plus, you need about twice as much insulation in your sleeping pad as what you would use in summer.