Sometimes the composer (or his students or associates) is not the final word in interpretation of his works.
I couldn't disagree more.
When it comes to interpretations, there are millions of different things people could do, and I agree that each interpretation is as important as another as long as it is well thought out and the conviction to see it through is there. I am not as familiar with Rachmaninoff and his contemporaries as I am with other composers, so am going to use the example of Gieseking with Debussy, who is supposedly the "final word".
The composer does have the final word; they wrote the music and they saw it in a certain way. It doesn't mean that this is how it
must be played and that any other interpretation is "wrong", neither does it mean that if the composer were still alive they would not approve of a different reading, it means that at the time of the last word of the composer, that is how they wanted the music to sound.
I cannot stand the majority of Gieseking's Debussy, and find it very cold and uninteresting. If anything, this means that I therefore don't like the intended sound of the piece, it doesn't mean that this wasn't how Debussy wanted it. Though in the Gieseking case, I'm sure it was Debussy's wife rather than Debussy himself (?)