Markus, I suspect you can claim to be the world's most erudite expert on the history of that kit!
I built it for the first time when I was about 12 years old. The one I bought was in a Pyro box, labeled "Civil War Blockade Runner," with the green hull. (By the way, in standard English usage ships don't have bodies; they have hulls. I gather your native language is German or Dutch. Do those languages use the same word for both English equivalents?)
You can work out the scale of the kit accurately if you start with my drawing on the CG Historian's website. (Don't rely on published dimensions; it's too hard to figure out just what they mean with reference to the model. "Length of keel" doesn't help you, for instance, because you can't tell just where the forward end of the model's keel is.)
Make a printout of the drawing with your printer. Down at the bottom of the drawing, forward of the bow, is a scale bar, marked in imperial feet. (I never put written scales, like "Scale: 1/8"=1'," on drawings. I have no way of knowing if they're going to be enlarged or reduced by some publisher.) You can rely on that scale; it originated with the Webb plans. Using a straightedge, draw a line between two conspicuous points that can be found on one kit part (say, from the tip of the taffrail to the tip of the figurehead). Using the scale bar and a pair of dividers or something similar, figure out how long that line is in the actual ship. Then measure the same dimension on the kit hull. Divide the full-size dimension by the kit dimension and you've got the scale of the kit. My guess is that it will turn out to be either 1/144 or 1/128 - the two scales that the Model Shipways kit has claimed to be. I'd be interested to know.
There is, of course, no absolute guarantee that the kit designers got all the proportions right. You might fine that the kit is on one scale in terms of length, and another in terms of beam.
The Round 2 website looks a lot different today than it did yesterday. The "Blockade Runner" and the Inpact kits are on it now. The "Blockade Runner" sells for $29.95 - not bad by today's standards (though Round 2, with no design or mold-making expenses, must be making a big profit on it). When I clicked on "Buy Now" on the Flycatcher and Avro Biplane pages, I got "The page cannot be found." But I suspect that will change in the next few days.
Long ago there were two similar-sized Clermonts on the market: one from Lindberg and one from ITC. (I don't think Pyro ever did a Clermont.) I'm pretty sure the Round 2 version is the Lindberg one (I recognize the box art) - and that's a good thing. It was a really ingenious kit. Originally it had an electric motor, and it's about the only motorized kit that I wouldn't mind building again complete with the motor. In the real ship all the machinery was exposed, and so it was on the model. The paddlewheels, scale gears, and flywheels turned, and the piston moved up and down in the cylinder. The electric motor was hidden under the forecastle deck, with just a little tiny plastic worm gear sticking out into the "engine room." Great fun. The only problem was that, as Old Timers like me vividly remember, a pair of 1960s AA-size batteries would only run the model for a few yards. But I guess Round 2 doesn't include the motor and associated parts.
The Southern Belle was a nice kit too - motorized, so the big, external cranks turned the paddle.
Don't waste your time on the Lindberg/Pyro Olympia. It was a pretty awful kit even when it was new. The old Revell one (recently reissued with wood decks and photo-etched parts by Encore Models) isn't up to the current state of the art either, but it's a whole lot better.
I agree with your ideas regarding the various paintings. The only really reliable contemporary views of the Harriet Lane seem to be the simple drawings by the designer, William Webb, and that faint smear in the photograph I referred to earlier in this thread. The magazine engravings on the CG Historian's website may qualify; those magazines sent artists around with the armies, and it's quite possible, I suppose, that the guy who did the drawings on which the engravings were based witnessed the capture of the ship. But they don't show much that helps the modeler.
That account of the hull being sheathed entirely with copper is obviously nonsense. My guess is that the person who wrote it didn't quite understand the source he was looking at, and didn't quite quote it correctly. It's entirely possible that the hull was fastened entirely with copper spikes and bolts, but sheathing it above the waterline with copper would be ridiculous. It would cost a lot of money and have no practical benefit whatever.
I'm no expert on CIvil War warships, but I've never heard of one with a steam-powered capstan. But I guess it's possible. I don't think there's any way to find out if the Harriet Lane had one. Every rendition of her that I've seen has had a horizontal windlass - but I suspect that's just educated guesswork.
I think the best approach to my dory problem is to make them out of either holly veneer or bristol board, shaped to match the actual hull strakes. But I haven't tried it. I've also considered investing in Micromark's new vac-forming machine, and forming the hulls over a carved master. That problem, fortunately, is a long way down the road. I haven't even started the model yet.