I don’t know where the idea came from that protein causes cancer, since protein is mostly not metabolised. The constituent amino acids in the protein we eat are mostly reconstituted into new proteins and used for repairing tissues and building new tissues. A small amount is deaminated; the nitrogen being used to control blood pressure, and the remnant of the molecule being converted into glucose to feed cells that require it.
The claim has been made that eating red meat causes cancer, but the problems with meat are supposed to be other chemicals found in it, not the proteins. But in any case, that claim has also been thoroughly debunked. The IARC, part of the FAO, published a report a few years ago, in which most of the so-called “evidence” they cited failed to back up their claims. (Dr. Georgia Ede has a couple of great lectures on YouTube, in which she discusses this.) The well-known oncologist, Dr. Thomas Seyfried, argues that it is the metabolic damage from glycation and oxidation caused by a high-glucose (carbohydrate) diet that causes all cancers.
Ketone levels are often lower on a carnivore diet than they are on a ketogenic diet. And those levels are lower than the levels seen during fasting. But the point of eating keto or carnivore is not to be “in ketosis” per se, but rather to be in a metabolic state where fatty-acid metabolism is prioritised over glucose metabolism, and a carnivore diet certainly promotes that metabolic state.
As for long-term concerns about staying in ketosis, I think they are largely bogus. We are so conditioned to think of carbs as “heart-healthy” that such a mindset is hard to overcome. As Gary Taubes points out, however, we used to consider carbs as fattening. They only became “heart-healthy” once Ancel Keys managed to convince everyone to fear fat. But from all indications from anthropological data, we evolved being in ketosis for the most part, since isotopic analyses of bones from archaeological digs indicates that our ancestors ate an almost exclusively meat diet. Agriculture as a human practice is only about twelve thousand years old, whereas our species evolved over two million years.