With respect to the down bag fitting into a Nalgene, I suspect the opening would be too small. Seriously though, yes, with the newer fabrics and higher rated down, you can get bags that will be comfortable to freezing that compress easily to the size of a Nalgene. Especially quilts. My Golite 20F rated quilt definitely compressed that small.
Doesn't look to me this way:
That's at least 4-5 bottles. Some people are having hard time judging the real volume objectively. 4-5 L is as small as I've seen. It's definitely small, but not Nalgene small.
Then the reviewer says he was cold at -1C:
As none of my first few trips with the Ultra 20 made it below 35 F (2 C) I was very happy with it. I had it on a variety of pad combinations and all was good. Then I had a very cold night high in the Sierra Nevada where it hit 30 F (-1 C) at 11:00 pm and stayed there. My water froze and so did I.
My experience with bags is that they're always about 5-10C too optimistic in their ratings. Sure, you can stuff yourself with extra clothes, but then it's extra bulk and weight you have to carry, not to mention these clothes can be dirty - I wouldn't want to put dirt inside a bag.
I too like room! You can get sub 20 oz fully enclosed 2 person shelters now in Cuben Fiber. Certainly under 2 lbs in silnylon. The Tarptent Protrail that I have can actually fit two standard thermarests side by side but the tent is claimed as a 1 person. It is bigger than many so called 2 person shelters and still weighs 26 ounces with all pegs and guy lines.
Yes, it's light, but it's not as roomy as a Hubba Hubba. I think your tent will not withstand any sort of wind either. If you go real light, you will have to sacrifice size and durability.
As far as 5 lbs between friends, it does make a difference on long mileage days.
You're forgetting you're getting more comfort and durability in return. On a multi day trip I'd be highly cautious about how safe my gear is, because weather can change quickly up there. What are you gonna do if winds and rain pick up, and you're 3+ days away from civilization?
The -7C or so I am relating to is for sleeping / around camp.
Depends how long you stay out. If just enough to make quick dinner and go to bed, then I suppose it can be OK. But if I want to stay up for hours to take shots of the stars, or wait for proper sunset light, then I need more than that.
Lighter weight loads, if possible, are also safer.
On the back, yes, but not in a storm or when you need to push your gear to the limits. On a hot, summer day, at low elevation in the trees, this is most likely not an issue. But most of my trips are somewhere in the alpine.