I agree with all of this—except the word “completely”—and I believe JRRT would have as well. The “Tolkien Professor” has done a good job, I think, of reminding his listeners that JRRT himself, when giving notes on an adaptation far, far worse than this one, was more concerned with preserving the spirit of the work than with preserving details not essential to that spirit.
On the other hand, I feel the series we have been given is not always perfectly true to the spirit of the work. But neither was Jackson’s LotR, which is nonetheless a masterpiece of a kind. And both are very close to the spirit, I think, and diverge only occasionally. A good case can be made that RoP diverges (so far) less often.