At (a) is the "undeniable" V9 chord with its direct resolution to I. At (b), perhaps the theorists' V9 without a root? I am willing to entertain the idea given the parallel to (a), but note that ^6 eventually finds its way down to ^5 before the dominant resolves. At (d), V9/V; I take this for a legitimate harmony because of the arpeggiation identified by the series of circled notes at (c)—von Wolzogen, by the way, includes these bars in his (thus rather long) leitmotif.
There are several "lesser" references to ^6 and V9. Here is one of them. This is again in the Prelude and involves a structural cadence:
The curtain rises, we hear the same music as the opening, including (a) and (b), and it seems clearly enough to refer to the Hall rather than to Elisabeth: she enters only at the end of the passage—but note that the very first figure she sings is a neighbor-note D5-E5-D5--at (e).
Near the end, she links herself and the Hall in the repetition of the opening passage -- quite literally, as she sings the melodic notes at (a') and (b').
The final cadence brings E5 very much to the foreground: see the three instances at (f). These are all internal resolutions, but just barely. The second is the most plausible; the first and third can easily be heard as direct resolutions with embellishment.
