Wagner's oneiric warning 

I prefer Böhm for acts 1 and 3 but the Furtwängler Act 2 is sublime.

Exactly! The slow use of extended dominants and pedals is undoubtedly of great influence on French composers of the early 20th century. We believe that because of the tonic pedal, directionality is heard between the first note and the second note of each pair of measures as appoggiatura and resolution, which makes us believe that the E flat you refer to is an appoggiatura of the D flat, as marked in the next measure. This is even more clear in the sus4 chords that follow. Thank you for your support and for looking at our analysis carefully!

Awesome, thank you!

It's used everywhere! The common note relationship is also correct but less accurate. C-Em or C-Eb are both related chords with one or two common notes. On the other hand chromatic mediant refers specifically to chords which as Kostka says are of the same "quality" (major or minor) and whose root note is at a distance of a major or minor third (for example C-Eb or C-A) Even more accurate would be to analyse these bonds with Neo-Riemannian theory. In that case C major is related to E flat by PR transformation.

I would disagree, as the bass motion from a neopolitan to the V7 is usually 4->5, but here it is 6->5, which is why it doesn't sound like a neopolitan chord. Movement by a tritone is a common progression distinct from the neoplolitan, which is very common in late romantic music. An example is at the very beginning of dvoraks 9th symphony second movement.

You can download them in IMSLP, they are public domain.