Hi,
I played the first and second movement before dropping the piece completely as I wasn't satisfied with the initial results. I remember having a REALLY hard time with the polyrhythms in the first movements and also with accuracy in hitting the chords and thirds in the triplet figures.
As for the rest, I'm sorry but I feel like I'm not knowledgeable enough to advice you for this particular piece. Don't practice slow, for me the outcome was horrible. Instead keep practicing hitting the right chords on full speed right from the beginning.
Hope that helps!
BW,
Marijn