one prelude is in E sharp minor according to them.
You can't really call any of his pieces flat out atonal, but I would say that around Op. 57 or 58 he starts to change over a bit. Even his last pieces, the Op. 74 preludes, have tonal tendencies to them, so to call them flat out atonal is sort of wrong.
The piano sonatas are a brilliant microcosm of his general output. Listen to all his sonatas in order (they are all very short, most around the ten minute mark) and you will hear his progression from very tonal (but still uniquely scriabin) in the first two sonatas, right through to the weird chromaticism of the last few sonatas.Most of them should be up on youtube if you don't have any cds of them.Enjoy!
Ashkenazy and Arrau are far from the great Scriabin interpreters, which are these three: Sofronitsky, Richter, and Horowitz.Sofronitzky married his daughter for crying out loud!
You must understand, just because something does not keep a certain tonal center doesn't mean it is atonal. The ones you have labeled as "atonal" really aren't, but they have a freer tonality. That is the case for many of his pieces from his later period.