I think you are overanalyzing all of this.
Most people wouldn't even be able to tell if the gauge isn't exactly to scale with the model, especially in such small scales. I suspect that 1:150 was settled on by Japanese manufacturers because it was a scale already used in other models. Btw, according to Wikipedia some Japanese modelers do use TT scale (1:120) on 9mm track which is closer to the prototype.
