extremely irritating in many ways...

Japanese War Bride is indeed a very good and under-appreciated movie. Sayonara is also an excellent movie. Montalban was a casting stretch but something like that was inevitable at the time. He put in an serious and honest effort. It is easy to criticize it from the 21st century. It is difficult to imagine any of the American Asian actors of the time pulling it off. If it was a couple of years later, they might have tried James Shigeta but he would never have projected the physical strength that Montalban did and that does seem to be a substantial part of the role.
Outside of Montalban, what is fake about the Japan of the movie? What was misrepresented? We didn't get to see much of it in any case as the movie was very dialogue and character driven.
As for Brando, I think you are in a clear minority. He did a lot in that role. Certainly, he didn't just deliver his lines, he built and grew a character. It is difficult to imagine another American actor who could have done as much with it or who would have been more believably cast. Who would you get? William Holden maybe but would his accent actually have been better? Glenn Ford? No. Heston? No. Montgomery Cliff? Obviously within his range but wrong image. Gregory Peck? No. Douglas, Lancaster, Hudson? No, no and no. Tony Curtis? For sure no. Paul Newman? Okay, he could do it but he would also change the character considerably and in a way you might also find "irritating". The other problem is that the movie was set up to be a BIG release and Newman was not yet a big enough name. So the choice of Brando was very understandable and in the end, his performance was well received even if you happened not to like it.

Great movie, great cinematography of Japan, great performances. Sorry for you that you can't enjoy it. Too bad, so sad.