Montbell Down Inner Vs. Alpine Light

Hi Martin! I have the Montbell, and at 7.4 oz it was a terrific investment. I have used it for the purpose you propose – to extend the range of my bag when winter camping. With the addition of a vapor barrier liner I was able to use my 18 oz Montbell UL SuperStretch #5 (the old style without a zipper) in the snow! The rest of my kit consists of a Smartwool ss T and ls zip T-neck pullover, a 10 oz zip T-neck polartec pullover and a 4 oz Patagonia windshirt. Besides the usual balaclava and shell w/ wool gloves, this is about all I need for temperatures down to the mid twenties. I also use Terramar silk longjohns, not so much for insulation but to keep the sleeping bag clean. The basic idea is to use several light layers rather than a couple of heavy layers. If you think you'll be encountering lots of damp weather, you might want to consider the BPL cocoon pullover, because a down jacket will absorb some moisture (insensible perspiration) overnight and might not dry out during the day; after a few days it wont be much of an insulating layer. That's why I use a VB liner with my bag: to keep the moisture out of the down. With care, you can use down and be light and fast!

The Alpine Light Jacket has twice the fill but a heavier shell fabric. The fill/total weight ratio is better for the alpine light even with the heavier shell.

I'm not sure why you are considering both as alternatives to each other, when you don't need the additional insulation of the alpine light it is too heavy, when you do need the additional insulation the down inner isn't warm enough and taking 2 down inners is heavier.

This was my fear, that the down inner wouldn't be enough, and that the alpine light would be to much. What insulating layer would provide that happy medium. Would it be the Thermawrap (synthetic layer)? This would be easier to launder in town, and would handle moisture better than the alpine light, but it probably has the same downfall of the down inner, being that it isn't warm enough. What would you suggest as alternatives?
I want to be able to stay warm in it in camp, and add a little extra to my sleeping bag in case I get cold while sleeping. Alpine Light seems like overkill, down inner seems inadequate.

I suspect the order in terms of warmth is this:
Thermawrap Jk < UL Down Inner Jk (< or =) Thermawrap Pk < Alpine Light Jk

The down inner is warm enough around or slightly below freezing as long as you are doing something. To me that seems to be a good match with your quilt. For being static you could additionally wear your quilt (might be difficult in the rain).

The alpine light is more suitable for activity way below freezing or keeping you warm on its own while being static around freezing.

Unfortunately I don't know any other down jacket with less than 4 oz fill.