Geoff, I can only tell you my experience.
As far as 58% protein turning to BG, it is ALL over the place. Bernstein, Rosedale, BS101, Primal Mind Primal Body, Think Like a Pancreas etc. As far as I can tell these books were not written 100 years ago but YMMV
First, I’d like to say that you’ve raised many excellent topics and I had quite a bit of enjoyment reading up on them. Furthermore, sharing personal experiences is very important and it’s largely why this forum as a whole exists.
Trouble arises when it’s unclear if someone is stating a fact or an opinion. When I encounter these situations, I do my best to ask questions to provide more clarity to myself as well as the rest of the forum. Sometimes my actions are misinterpreted, but my ultimate goal is to help others make decisions based on accurate information. In return, I would expect others to question my thoughts and opinions just the same.
My comments below are my basic personal assessments of each book or source you mentioned above. I included quotes from three of the five sources you mentioned. I do not consider the other two sources to be qualified medical professionals, but I acknowledge the value their books provide to many people.
1.)Bernstein specifically states in his book “Diabetes Solution: The Complete Guide to Achieving Normal Blood Sugar levels:”
“As much as 36 percent” if the right conditions are met and cites “If blood sugar descends too low, if serum insulin levels are inadequate, or if the body’s other amino acid needs have been met.”
He also states in his book that “The average non pregnant, sedentary adult with an ideal body weight of 150 pounds” needs about “69 grams of pure protein” to prevent protein malnutrition. He also goes on to say that “Most physically active people should consume at least 1-1.2 grams of protein per kilogram of ideal body weight.”
2.) Rosedale recommends roughly 1g of protein for every 1kg of lean body mass. On his website, he mentions that some people need “A little more,” and lists males, people who exercise, are unusually muscular.
3.) Jenny Ruhl has been called on this forum many times for citing outdated studies to support her opinions. While I believe her books are great motivational tools, I do not consider her opinions to carry any more weight than yours or mine.
4.) I have many similar thoughts about Nora Gedguadas: great motivational books, but she lacks credentials to be taken seriously as a medical professional.
5.) On page 51 of “Think Like a Pancreas,” Gary Scheiner states “Protein’s effect on blood sugar is minimal. One exception is when very little carbohydrate is consumed.” He even uses the words “When carbohydrate is present in a meal or snack, protein has little to no effect on the blood sugar.”
In conclusion: this is an incredibly complicated topic and the medical community is extremely divided in regards to what is "too much" or what is "too little." Consequently, we have no chance of ever achieving the answer here in this thread.