These images do not look like Photoshop. The color gamut of the original negatives would probably be outside of the gamut of RGB. What has happened in post-processing is that the contrast and saturation have been exaggerated (is it too much, is that all you're really saying?). I've pulled some of these very same images into Photoshop and reconstituted them into color images.
What you've described in so many words is quite simple. These are three color channels, overlaid on each other (Photoshop makes auto-aligned layers very easy).
That's it. And basically that's what the three lens lantern projector that Prokudin-Gorsky used to present his images did.
The original source images, due to their being recorded on plates or film, probably have a wider gamut, due to the greater latitude of the analog medium, that most of the digital cameras produced today.
While we can't say for certain that this post-production has yielded an image of reality that corresponds exactly to the colors and lighting of those scenes 100+ years ago, grass is only so green, some historic dress survives, and there are archivists who study the deterioration of dyes, and while I don't know if the aspect of clothing has been studied in the production of these images, how can you say that these images are not an impressive reconstruction?
Basically, your argument boils down to this point. It's as if there were an ancient sculpture in three pieces. Researchers glued those three pieces together. You're bitching about the fact they used modern tools to put them together.
If you were to take these lantern slides and project them in the very same projector used by Prokudin-Gorsky onto a wall, I think you complain when a digital photograph was taken of them and displayed on the Internet. You are looking for things to nit-pick about.
What you haven't demonstrated an understanding of is how color film has always worked, since the very beginnings in experiments by Maxwell. There are layers of monochrome film, because that's what a sensitized silver emulsion is, is monochrome, there is none other. Above each of these layers of monochrome emulsion are dyes acting as filters, generally three, which filter all but 1/3 of the spectrum, giving us a red layer, a green layer and a blue layer. The film when processed removes the dyes which didn't get exposed (or did, depending on whether it's print or reversal film).
Digital cameras are the same, there is a light sensitive layer, which again, is entirely monochrome, sensitive to all frequencies of light. Above this layer is a layer of filters, called an Bayer array, which is two parts green, 1 part red, 1 part blue. Each light sensitive site has a filter of some color sitting over it. When the sensor is exposed and read out, the camera software processes it into a true color image by interpolating the color information onto adjacent pixels (demosaicing).
Finally, what you fail to demonstrate is why we should care where the color space and tonal separation come from. These images weren't shot on an overcast day, there's plenty of contrast present in most of these images, as they were shot on sunny days. Film emulsions in 1900 weren't very fast, the equivalent of ISO 25 or slower. You can get an idea of the duration of the exposure by looking at the silkiness of the water in the photos that were shot near water.
What I keep coming back to in my mind, is why you think that being produced in Photoshop diminishes or alters the reality of these scenes.
I too am sick of seeing candified images with off-the-charts color saturation, produced that way just because it can be, because colorfulness is compelling.
I don't think the images fit the definition of over-colorful. It would surprise me that the Library of Congress, whose mission is the archival of material, would make no allowance for the accuracy of the colors presented, however I see no allusion to the research, so perhaps these bear no resemblance to reality. These images come close enough that a contemporary viewer would be awed by them, and a modern viewer can better appreciate and understand the world which bore them. It takes true artistry to convey emotion with a monochrome image, so I'm glad we have these color images to convey more of the feeling of life in that era.