I think you're on the right track. Down is perfect for low-activity warmth. And since you will using it as a layer (wearing a jacket over it), it's just a matter of buying a down jacket that will fill up but not overly compress within the space you have. If you have money to blow, you could go with Feathered Friends or Go-lite brands. But I don't know if you need to worry about weight (and the high price) unless you are facing a huge hike in.

The only downside to this approach is if you ask the down layer to insulate you on the walk in. Any layer that keeps you warm while walking won't be warm enough during sitting. So I think you need to consider keeping the synthetic layer and adding the down layer.

A couple other points: a vest/jacket that has a collar seems to really keep the heat in (probably lots of heat loss around the neck). A down hood is also great, but probably not applicable to hunting situations.

I actually use the vest more than the jacket, mostly as an insurance piece of gear in spring/fall when it is hard to guess how low the temps will go. For the teens, I'd go with the jacket unless it won't fit under your coat. Both are very light for the warmth they provide!

-jamie