Cindy, interesting to see your ideal pack from 2016. I, too, have come up with a set of criteria,
(not item-for-item identical to yours, but similar) I would like to find in a pack.
I'm not wedded to a maximum size, as long as it holds what I take (23L) with a little margin. It can be bigger as long as it has external cinch straps to tighten the load. Being storable under the seat (<35L) would be a plus, but, in reality, I have never found stowing my pack under the seat to be necessary.
I definitely want it have a single large compartment that opens like a book, stowable backpack straps, a sternum strap, and external cinch straps. I would call those non-negotiable.
I used to think a rectangular solid shape (I think that's what you meant by rectilinear, but that's not the correct term) was important because it maximizes the capacity within the airline maximum dimensions (bags that taper or that have a thick pocket over only half of the footprint are volume inefficient). Now that I am well within the regulation dimensions, that doesn't matter. For those who can't control themselves and constantly press the limit, a rectangular solid pack would be important.
I do like to have one external pocket to hold my passport, wallet, neck pouch, and belt while I go through TSA security.
I found that with a waist belt, I can completely loosen the shoulder straps and support the weight of my bag on my hips, but my bag is so light, at about 13#, that a waist belt is not necessary for me. I leave the waist belt at home. For someone with a weak torso, a waist belt might be needed and would nullify the "my back and shoulders can't take a backpack" excuse.
I, too, would like less than 2 lb, but I grudgingly accepted an additional ¾# to get cinch straps and a sternum strap. I felt the additional weight was worth the comfort. I went from a 1¾# OPEC bag to a 2¾# eTech W/E convertible Jr. to get the feature I wanted.
I recently acquired a Hynes Eagle 38L backpack. It has all the features I want at only 1¾#. It's advertised capacity is 38 liters, but the advertised displacement (LxWxH) is more like 33L, and I think it is really more like 30L. I think it would fit under a airplane seat, so it might be just about ideal for both of us.