How widely accepted is this?

Tl;dr This is not accepted or at least the main thesis. Werner appears to be reinterpreting history (badly) as a conspiracy based on flimsy evidence. As far as smoking gun confession (which he thinks he has, but not at all), all the evidence shows that this was very unlikely. It would be like saying that currently (Aug 2022) the Federal Reserve and all other central banks induced inflation to produce a recession and not taking into account Covid, war in Ukraine, and the supply shocks.

So a few things about Richard Werner. He is an academic or was one. He was a visiting researcher at the Bank of Japan (BoJ) in the early 1990s. He wrote Princes likely toward the end of the 1990s or early 2000s as it was released in 2002/3 (his Wikipedia seems suspect because his publisher says 2003). During the mid 1990s he was an economist for a private investment firm. So this might be critical for understanding his motives.

There are only three criticical reviews of Princes (book) that I could find and they were not positive: Smitka 2005, Farber 2004, and Myung 2004. The criticism overlapped in that Werner presents flimsy evidence, not accurate history, and faulty economic theory presented as facts. Ferber (2004) writes that it seemed way too personal. The book was meant for a wide audience as Werner meant to be clear, but also academic. He ends up being muddled and seemingly creating a narrative. I found many either inaccuracies, poorly contextualized facts, or many omissions that appear to be embellishing in service of a narrative. By the time Werner published Princes many studies were already part of the mainstream about window guidance and monetary policy, which Werner does not seem to employ correctly or dismisses. In addition many of his sources seem to be misinterpreted or misused. Some of news articles aren’t identified, which makes it hard to tell if he used them correctly, but based upon the context he appears to be putting words in people’s mouths as they were talking about different things. So Werner sounds more like a conspiracy theorist than an objective scholar. There are many questionable are in the book and if I had to estimate, it would be at least 30 questionable areas. Some examples:

• Werner mentions Hjalmar Schacht and calls him the “credit dictator” in 1920s Germany. There is no background other than he was just a mean bully on the economy of Germany for the sake of something. In actuality, it might be necessary to know that Schacht was president over Weimar’s Reichsbank during post-WW1 inflation due to war reparations. Schacht was trying to negotiate reparation payments with the Allies and get Germany out of inflation. He was also Hitler’s banker, so there is that. Yet Werner does not cite a source, nor is there any mention about in writings in English. Werner is German by the way, while I’m not a German native I could not find information in German either. So it might be that there was this sentiment about Schacht, but it needs context.
• Werner insinuates that General MacArthur threatened with the death upon Japanese officials like Nobusuke Kishi (grandfather of assassinated former PM Abe) and Emperor Hirohito for allegiance to the United States. Werner tries to makes this part of a chain of reasoning how US capitalism infiltrated the Japanese, but it really does not make sense and just ignores everything that was going on at the time. It seems like he was insinuating that the apparent suicide of one of the ex-PM (Prince Konoe) during Occupation proved this, but this might be dubious. Werner hid this in the notes section. So the connection is actually weak.
• Jun’ichiro Koizumi, PM 2001 - 2006. Werner tries to rope in Koizumi as part of this BoJ conspiracy, but completely does not contextualize that Koizumi was talking about different reforms. For full disclosure, Koizumi was educated in economics in England circa the 1960s/70s. The reform Koizumi was talking about the Fiscal Loans and Investment Program (FILP), or the Post Bank. The politicians and bureaucrats were using the savings deposits and insurance policies as pork barrel funds to buy off votes in rural areas. This was impeding monetary policy and forcing the Post Bank to buy government bonds keeping them cheap. So this was impeding growth and creating a dysfunctional economy. So Koizumi was not necessarily talking about the exact same thing and it was much later beyond the bubble burst after economic stagnation had become normalized. Werner appears petty accusing any reform as a US capitalist boogie man much in the same way that any help to the poor is Marxism/Communism. Koizumi appears genuine in trying to strengthen Japan. His bold leadership (even as a populist) is to be objectively criticized than character assassinations.

Some other criticisms to mention. The book is very poorly edited. It reads more like a bad spy novel. I find that Werner keeps repeating himself and then slightly different details each time he repeats himself. He poses questions and rambles for a while, and then randomly at times returns to answer the question like it was obvious. So he plays tricks as he accuses the princes of unclear jargon and hiding, but he might be throwing rocks from his own glass house.

About the documentary. Werner’s book is very creative at bending facts in rather artful ways, but the documentary is not subtle. In fact, I think the opening line should be taken very seriously: ‘based on the book: Princes of the Yen’. So the book is rather dubious, then documentary is even more dubious. I would parallel it to The Last Samurai is based actual history, but has a lot of creative liberties that it is mostly loosely just fiction based in history. So there are many things that were not mentioned in the book and were likely made up as a narrative. One example is “non-GDP based loans”. The documentary gives a definition, but this is not a real term and at face value makes no sense. All internet searches lead to writings about the documentary. No investment or economics dictionary has the term.

So, I just watched a documentary based on the book "Princes of the Yen" and it argues with many quotes from Japanese higher ups that the "window guidance" was the critical force behind Japanese economic "miracle", in which the Bank of Japan gives quotas to banks how much they should lend and to what companies under low interest rates.

So “window guidance” (窓口指導 madoguchi shidō) is not really what it seems as described here, and I’ll explain later what it means. Just iterating the definition, while “guidance” can imply “control”, it would preferable because banks flouted “guidance” in various ways. More later.