Just some observations:
In Rachmaninoff's own practicing, he used the French method of practicing passages:
Using a metronome set at a slow tempo, play the passage a few times. Then advance the metronome a notch or two so that your muscles hardly feel the difference and practice it a few more times. Then advance the metronome another notch or two...
By the end of a practice session, you will have reached your tempo limit. The next day, start again at the slow tempo and gradually advance the metronome. After a while, you will be able to perform the passage faster than you need to. But never stop the slow-tempo practice.
Rachmaninoff felt this was the only way to develop secure fingers and memory. But it is VERY time-consuming.
Perhaps a better method was the one advanced by my own teacher, the late David Bradshaw:
Divide the notes into groups of three or four. Play the first group quickly, then stop. Look and mentally prepare the next grouping, and when you are ready, play it quickly. Keep doing this until you reach the end of the passage. Then change the grouping so that you are stopping on a different note. He learned the entire concerto for a performance in a week using this method.
Parenthetically, I'll note that the late Gina Bachauer studied this concerto with the composer. He showed her all kinds of different hand divisions and fingerings that made it a bit easier. She later tried to interest publishers in bringing out an edition which would incorporate these new ideas, but no one was interested.