David,
Glad to be of help. If you think making shoes from the other side of the world via e-mail is rough going, you should try making authentic replicas of 250 to 400 year old archaeological shoe fragments--that's what do mostly. Cheer up, at least there are still folks alive who can help you with this kind of boot. Unless I hold a séance, I'm stuck with the books mostly.
Now, down to cases...
I don't need to tell you to tread lightly when you write Goudie's daughter, nor say anything that might scare her, but I'd think you'd be alright saying you'd like to purchase a spare pair of those special lasts in your size, and maybe any in-process uppers, because you desperately loved her father's boots and want to try to make yourself one pair. Don't sound even vaguely like you're contemplating going into the business--you just want them for your own use, right?
The lasts are the basis of any footwear, and emphasizing their importance cannot be stressed strongly enough at this phase. If you want *that* boot, *that* look, *that* fit, you really need to start with *that* last, especially in this case where it is so extreme and unusual. You're not likely to find another one to mackle-up, or make do. You need that last first off. Second choice would be to get a pair from the other firm, if it's the same design. Third choice would be to ask them what is the model number, and then contact Spring-Line Last Co., Moulten Park, Northampton [UK], and see about having a pair turned in your size. But, I'll bet, if you approach them right you ought to be able to get an old pair.
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"asking beginner's questions about first principles. Is the bottom of the hill boot rigidly constructed (by riveting) to keep the
spring there? Would so much spring otherwise collapse in a boot stitched rather than riveted, or would skilled lasting and choice of materials alone
enable the spring to remain fixed regardless of clenched rivets being there or not?"
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I'm not sure how to weave these features together in a simple explanation, especially since I've no first-hand experience with that boot, but I'll try. Toe-spring is usually increased in any boot that has rigid, or semi-rigid bottom, so you can walk more easily without having to or being able to force the bottoms to flex. Wooden-soled clogs are an example--they won't flex, so the toe-spring is raised way up so you can sort of roll along. I think the high toe-spring in these boots is to overcome the rigid bottoms. Now the bottoms would still be nearly rigid, if that thick, and stitched. The problem there, however, would be stitching that much material, which was probably why riveting was adopted for them. Besides, it's faster, cheaper, and more waterproof. The "why" and "wherefore" are beyond my ken I'm afraid in this case. My best guess is that the exceedingly thick leather soles were adopted for wear/protection/durability first, then the toe-spring was raised via the last design, [even higher than a wooden-soled clog, for walking on cobbles and pavements], because these were to be worn on hills, and one wouldn't want the toes to plow into the dirt with every step ascending hills. Subsequently, riveting is just the logical construction method the design dictated. What holds the toe-spring up? It's mostly the last-shape and the upper, or to be blunt, they're made that shape, so they pretty much stay that shape. As I said, I'd wager even *if* they could be made that thick and stitched, it wouldn't compromise the spring much at all, so you wouldn't flatten the toe-spring out or loose it in wear--not in a boot that heavy.
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"Is it right that the clogs are so well sprung because the foot cannot bend the rigid wooden sole, so the sole must be sprung to the
extent that the foot would bend to freely if it wasn't so constrained?"
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Yes.
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"Is a shoe last then straighter, but since the material of the shoe is more flexible this straightening is not a problem, but a feature giving a neater
appearance?"
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In general "most" mass-production shoe lasts are too straight, as you say, because it is: 1) easier and more economical to design and make shoes on them, 2) the majority of people can tolerate that shape in commercial footwear for everyday wear [it's a "problem", but they live with it], and 3) our idea of a "good fit" comes from a totally different philosophical premise than the shoe manufacturers who patronize the last designers/makers, etc. It's a long and grim tale, so I'll spare you the details, but suffice it to say, the footwear industry "discovered" toe "swing" as a fit variable in shoe last designing for mass-production c.1880s-90; learned how much was too much/not enough, etc. c.1900-10; experimented and got it generally about right in the 1920s, and '30s for mass-produced lasts/shoes; then after WW II, promptly "forgot" it again it seems except in children's shoes. Work boots, and other specialized types such as military combat boots, usually have more "swing", that is they're more anatomically twisted left and right in the bottom shape.
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"And if I use the exact tracing of the foot for the last bottom have I come close to including the natural swing, or is the swing in the hill boot more inward yet?"
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Yikes, that's a tough one--if it were but that simple... When a bare foot is standing dead flat on the floor, as it would be for tracing round, the great toe is at it most medial extent [in most cases]. So a last swung to that degree should be sufficient for nature. I observe that in gait, however, *some* person's great toes actually "grasp" out medially further still, if the footwear doesn't restrict or pinch the toe.. As the great toe is raised up off the floor, however, either by muscles or by toe-spring, it moves not straight upward in most cases, but upward and inward [laterally], so it might not need so much "swing" with that huge toe-spring. I just can't say. Experiment by looking at your own bare feet and watching what happens as you raise your own toes. Some people have bunions, or a laterally-tilted joint at the great toe, so their toe is bent inward towards the center-line of their foot. This person would naturally not need as much "swing" in their last. This is now moving dangerously close to trying to teach custom boot fitting via the internet, which is impossible. If you only aim to make yourself a pair or two of these boots, again, I suggest you just try to get the Goudie last, which obviously has plenty of "swing". Or, if that fails, maybe she'll do you a bottom tracing, or insole pattern off the last to guide your last-making efforts as regards shape [i.e. "swing"].
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"Should the backs be shaped somehow before lasting (blocked) or will this formidable material shape during lasting so that the back is not just straight up and down?"
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Well, I use 2.5 mm. uppers, and if you soak them it water before lasting, with a bit of heave-ho, and some hard lasting, you can usually get them pulled down to the wood. A lot is in the patterning, especially with no back-seam/heel-curve. If the center-back fold is angled wrong, they'll never come close to the back of the last. Experiment with the patterns, if you can't get Ms. Goudie to send you uppers [or her pattern]. You've got not only your 3 mm upper, but who knows how much of a stiffener or counter around the back too. I guess the easy thing to say at this point is, you'll never get as close or as "good" a fit in the heel with a one-piece quarter as you would a shaped and seamed one, but if this is an integral feature of these boots, obviously it's okay that way. My guess is too, that a little looseness in the quarters won't hurt, so your heel can slip up and down a bit--you do wear very heavy socks with these, right?
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"Do I want as near a Goudie style as I can, and learn to use rivets, or do I want to apply the waxed end making I have so much enjoyed learning about,
with all the welts and whistles that go with it to make a stitched variant?"
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These are philosophical questions that only time can answer. I will say this in closing though, I haven't seen many [any?] boots with bottoms that thick that were stitched, so you might be stuck with riveted--but hey, have fun. You can always make another pair with rivets, or even wooden pegs
. 
