Thanks. Due to some special circumstances (the flu claimed ~20% of our student population and the entire language department over the course of a week), school was cancelled. I intend to get close to or finish the Valkyrie in the five days I have, which would include construction of the lower nacelle and installation of the electronics.
Concerning the weight:
All components, as measured by a kitchen scale (don't have a bathroom scale):
Airframe, currently: ~1400g (see post #19, parts measured individually)
+ 6 sheets of foam+glue ~800g (estimated, foam is around 100g/sheet, less with the paper removed)
4 ESCs: 319g
4 Batteries: 1145g
Retracts, legs, bogies: 443g
4 EDFs: 744g
Servos: 112g
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Total: 4963g + other stuff* (expecting at most 500g)
*wiring, retract door sequencer, receiver, etc.
So it looks like I overshot my goal by at most 500g, or 10%. Not ideal, but certainly not the end of the world, as this was a rather optimistic estimate. If I had to point towards a reason for the large discrepancy in weight between the two craft, I would look towards differences in the power system and functionality of the two craft.
Wall of Text incoming, TL;DR at bottom
The main difference in weight would be caused by Battery selection. I am using the equivalent of two of your batteries (4 x 5000 2s), while you are using six. I don't mind short flight times, especially with the stress and anxiety that flying such a large and expensive plane puts on me, and the lower weight means that I can have less support structure, which means even lower weight. The decision to use only four batteries was also influenced by budget: I already had two of the batteries and used them in a large RC truck, and so buying two more was cheaper than getting a whole new set. This does however mean that I'll be putting much more stress on the batteries, but that risk is relatively tame, as the packs are rated to 50C and my ESCs will blow well before that. This is, however, irrelevant to the point, as your estimate was 6.8+ kg WITHOUT the batteries.
Another factor is the extra features. Your Valkyrie has a functional canard, folding wingtips, and drag chute. These require extra servos, extra wiring, and four 850mah batteries (as shown in the Q&A), as well as multiple receivers. If I had to guess the total weight gain from these features, I would say around 600 to 750g, with the wing fold servos taking up a significant portion.
Your Valkyrie's retracts are also much stronger, and therefore heavier. The retract units, oleos, and tires must weigh more than mine to support the weight of the plane, because they must be made out of higher-quality materials (metal in this case), as well as being beefier in general.
As far as weight regarding construction methods is concerned, The former-skin method is not as heavy as you might think. The formers, as well as one side of the skin, are stripped of paper (where I expect that paper remains on both sides for a box-style fuse section) which significantly reduces their weight. Additionally, the total area of foam required to skin the formers is less than a box-style fuse, as it follows a smaller, curved path. This also applies to the wing and wing ribs, as they don't benefit from the extra structural rigidity in the specific axis provided by the paper. As far as glue mass goes, box fuses will have a roughly constant amount of glue per unit of length, whereas a former-skin approach will not (with the glue being concentrated around the formers). I also don't glue every part of the former to the skin: the skin is pre-formed by bending it until it holds it's shape, at which point it is secured with an amount of glue based on the curvature (parts that curve more will tend to bow back over time, and need to be more secure than relatively flat parts). As a result, the amount of glue used in the construction of both is likely not that far apart. I have yet to do any proper experimentation, but I think that two equally-sized fuselages: one box and one former and sheeting, would have around the same weight. And again, this applies to all other areas of my plane that use this: the wings and center wing section included.
Also, I don't intend to paint anything except the details (USAF Logo, tail number, etc.). This will save considerable weight on such a large airframe, and I'm willing to sacrifice longevity and a glossy shine for weight.
TL;DR: The weight is lower than one might expect because of the lack of extra features, low amount of batteries, surprisingly light construction method, and lack of paint.
Edit:
Although being very similar in size, the two craft differ in just about everything else: weight, construction method, complexity, and power, to name a few. This can be expected from two different people with unique goals and ideas of what each plane should turn out to be, and the differences in the two B-70's are a result of that.
For me, this was a large-scale testbed of the former/skin construction method which would help me learn how to effectively and efficiently use it for future builds. It was also my first venture into ducted fans, proper retract units, and large planes in general.
I'm finalizing CAD of the lower nacelle unit and plan to have it done by the start of next week, unless the cough I got from school ends up being the flu