1. Hiragana/katakana can be learned in a week or two using flash cards and spaced repetition. It can be mastered through reading Japanese text for a few months to the point where you stop thinking about it. You don't need a $77 book to learn this, its just brute force memorization. I didn't know any other Japanese going in besides this.
2. Full immersion while ideal is impractical for most people interested in studying this language. You can still give yourself full immersion while learning anywhere in the world by using Japanese learning resources and limiting your English use to the minimum necessary (dictionary lookups, explanations for particularly troublesome concepts).
By the end of MNN 1 going into MNN 2 I swapped from a JP -> EN dictionary to a JP only dictionary. If I didn't understand a word from context in the book I would look it up in the dictionary, if I didn't know a word in the definition I would look that up and so on until I understood using only Japanese.
>I didn't know any other Japanese going in __besides this__.
I think that's where I was hung up. It makes total sense to first start with learning hiragana/katakana with whatever preferred method, then move onto something like the book you suggested. Rather than just starting with the book you suggested. And, I'm sure that point is obvious to many and why you left it out. But, as someone who only knows one language, it wasn't as obvious to me.
One more note that you may or may not be aware of:
Culture and language are closely intertwined, they drive each other, and Japanese is certainly no exception to this.
Japanese isn't spoken as literally or certainly as English is, especially to strangers. They use this system called "Keigo" which you'll find translated as "politeness" but that doesn't really completely encompass the idea. It is just a way of speaking in certain situations that covers your bases. Japanese is a language that is often stereotyped as needing to say a lot to say a little and this is often true.
Its useful to try and learn this intuitively. Hear and see it used often to the point that you just know the idea being communicated. Its difficult to translate many of these concepts to English because of how outside of our cultural sphere they often are (which is why I believe trying to teach them in English from the beginning is a fools errand, they must be learned contextually).
2. Full immersion while ideal is impractical for most people interested in studying this language. You can still give yourself full immersion while learning anywhere in the world by using Japanese learning resources and limiting your English use to the minimum necessary (dictionary lookups, explanations for particularly troublesome concepts).
By the end of MNN 1 going into MNN 2 I swapped from a JP -> EN dictionary to a JP only dictionary. If I didn't understand a word from context in the book I would look it up in the dictionary, if I didn't know a word in the definition I would look that up and so on until I understood using only Japanese.