Well, you covered most of the high-lights.
Thick bladed pruning saws don't work that well. Any saw needs a stiff blade to cut effectively. A single mounting point means a thick, stiff blade cutting away large amounts of wood. Generally wasteful of energy when cutting to make two pieces.
A bow saw is normally used to lengthen the metal in the blade 3-5 times with the bow providing a fair bit of tension on the blade to stiffen it. The body of wider blades can bind easily in rough cuts due to the wood fibers rubbing on it. So a thin blade is needed with a fairly wide kerf. But, more body is needed to keep tension on the thin blade and to provide clearance around the piece being cut (6-8".)
From an engineering standpoint, the a partial bow saw, a triangle shape as you have designed, is the best for form stability and strength. The pieces are not independent of each other, hence are locked in a bit better. This is the old SVEN saw design, too.
A full bow saw is three pieces plus the blade in a rectangular arrangement. However, it lacks any inherrant stability, but provides greater clearance.
The average cut stroke for an adult male is about 20". Some more, some less. So a blade of 21" (counting the dead area near the attachments) plus mounting points gives about 23". I think the Sven saw is a bit short at 21". A carpenters saw is a bit to long at 32". But, a lot is what you can get used to.
The tubular design is good, but it is too narrow to hold the pressures need for good strength. Idealy, both the pull, and push stroke should be used for more efficient cutting. The push will increass the tension needed on the blade to prevent blade flutter (and binding on the wood.) Pulling will increase the tension on the the other end. There is a LOT of pressure needed to remove the blade flutter and compensate for the dynamic stresses. I would guess around 200-300 pounds of static pressure to hold a blade to a few thousands of an inch of blade kerf while cutting a piece with a constant 15 pounds of force. This is where the smaller 1/2" diameter tubes fail. You will get a lot of blade flutter and binding, unless you drop the force to 4-5 pounds. You won't cut very fast with it.
Most blades today are electro tempered, meaning only the teeth, often only the tips, are hardened. This lets the body stay softer and they can be drilled for attachment pins. Of course, some tensioning mechanism is needed. The SVEN saw uses a wing nut and a longer bolt as a blade mount on one end.
The aluminum, frame can be made from 2, flat, 1/16" thick pieces of aluminum and one 1/8" piece with a couple rivets around the blade mount, all about an inch wide. A couple pieces of aluminum can be added above to allow the body pieses to lock against when tensioning the blade. Better is a hollow tube, flattened, like the SVEN saw, though the "u" shaped channel is functionally equivalent to two pieces... Since most of the strength is in the inside corners(compression) and outside corners(tension,) the handle/body pieces can be drilled out, reducing the weight by 25-30%.
You could make a bow saw. or, simply buy the SVEN saw and modify it. In colder months, I usually just bring a modded version of this saw. With mods, these weigh around 10oz.
Since there is a lot of pines up where I go, I usually open the kerf a bit and keep the blade sharp, though a grinder is needed for the hardened teeth.