The "Rachmaninoff plays Rachmaninoff" CD with this piece on is on its way - I only just ordered it

Having said that though, I've changed my mind now. The D# isn't doubtful, nor does it "sound better". It's the only reading that makes sense given the structure of the piece.
The 2 outer sections are based around the falling minor 6th motif in the bass, which Rachmaninoff helpfully outlines for you at the very start (well, he was quite young; I can't see him being that obvious in later works!) He establishes it in c#, then repeats it in E and g#, making up an ascending c# chord. The chord on the 2nd beat of each "transitional" bar, 5 and 6 in the first section, is anticipating the next iteration. In bar 5 we have a B, C# and G# and we are moving from c# to E, so if we throw in the "destination" E (which Rachmaninoff makes a point of never doing), we get the chord of C#min7. In bar 6 the harmony is moving from E to g# and the equivalent chord is E
maj7 - the D# here is the 5th of the key we are aiming at, as the B was in bar 5. Making it an ordinary E7 by putting in the D natural makes a nonsense of the progression.
As far as pedalling goes, well, you're the teacher, I'm just an interested amateur

But since I've stuck my oar in I might as well stir it about a bit. Assuming you're on an upright without a sostenuto pedal, I'd say pedal on bass note changes, half pedal on treble chord changes where there's a sustained bass note you can't hold. I'd be more inclined to worry about keeping the bass line going than getting a clean sound in the treble. Think of it as bass (falling minor 6ths) plus accompaniment. If you've a sostenuto pedal you can do all the pedalling you like in the accompaniment, the tempo leaves plenty of room for it. Where the phrasing is in 2-bar sections in the
agitato I take the pedal off entirely in the second bar of each block.
Of course, given that I'm not really qualified to talk about this stuff, you're quite free to ignore all the above.
