In the concentration camps, Nazi-affiliated physicians did do terrible, inhumane, "fringe" science on prisoners. The most famous of these is the notorious Josef Mengele, who did things like collect the eyes of twin children for the hereditability studies of his mentor, Otmar von Verschuer, and to participate in wholly unethical studies that involved deliberately giving people diseases, amputating limbs, giving transfusions, and so on. There were additional Nazi experiments that involved things like subjecting prisoners to freezing conditions, chemical weapons, and other damaging and often fatal circumstances to measure their responses.

But here's the thing. All of this was done in a very unscientific setting with very problematic methodology. The people doing the work were not, on the whole, the best and the brightest — they tended to be the most sadistic and amoral. This does not result in good research. The Nazi data is, on the whole, junk. It turns out that the compromise is not between "do we do unethical experiments to get good data" or not — the Nazi approach to science turned out to be junk science as well. Which makes it all the more depressing. The fact that it was largely isolated from scrutiny and engagement with the scientific community allowed it to drift far outside of good practices, as well. For a very vivid description of this, from someone who was forced to work on Nazi science during the war, see Ludwig Fleck, "Problems on the Science of Science," which describes how the bacteriological laboratories at Buchenwald worked and how ultimately it produced just junk.

I don't know if they worked on "revivification" studies, but such things were done extensively in the Soviet Union before and during the war. The short answer is: you can keep heads/limbs alive for a short amount of time separated from a body. You can even take a recently-killed organism and bring it back to life. (We do this with humans, sometimes, by re-stimulating the heart — hence the "I was dead for X minutes" stories of people who were "brought back to life" in the Emergency Room.) Inevitably there becomes a limit to how much damage is done (without continuous oxygen delivery, the brain suffers permanent damage within minutes), and how long a partial organism (or organ) can survive in the absence of major body functions (depending on the circumstances, a while; there are today ways to keep a full body alive for a very long time using machines). The work is disturbing. Is it fringe? I mean, in a way, yes, but in a way, no — this is how organ transplantation can work, for example, and how bodies are kept alive even after brain death (a contentious medical issue). But it is not miraculous in any way. Doing it on non-consenting human beings just for the sake of trying it out would be deeply unethical. The Soviet research was on dogs. If you want to watch a very disturbing Soviet film where they do this kind of work on dogs, have at it, but I warn you, like much animal research, it is not pretty. I don't know if the Nazis did research of this sort, on humans or animals; the description sounds like an idea of Nazified Soviet research to me.