If it is so cold that you will want more than your baselayer + down sweater under a hard shell, it will be below freezing, so a hardshell will not be needed. even if it's windy, you will essentially be wearing 4 windshirts (2x inner fabric + 2x outer fabric), so wind won't be an issue.
I wouldn't suggest hiking in a down jacket under a hardshell, great way to have a sopping wet down jacket after a few hours or at least days.
so i would add a thin fleece(vest).
Even over fleece I would choose windshirts or softshell outers, and perhaps carry a lightweight hardshell if there is a chance of wet precip.
The one exception would be Nikwax Analogy (Paramo/Cioch), as it is functionally equivalent to midweight base layer and windshirt, that just happens to be waterproof.
The other exception would be if you are wearing VB clothing, then ofcourse you can wear down with a shell over it without issues.