The chord that goes on the fourth and fifth bars of my attachment above looks like a Neapolitan chord. The excerpt is the second movement of Beethoven's Symphony No. 3: Marcia funebre Adagio assai. (The C at the end of the fifth bar as a passing tone) In fact, the chord progression appears as if it is I - N - V6 in C major. (C - D♭ - G/B) Am I right?
Yes, it's a Neapolitan chord. Because of the arpeggio, it's not in the usual position (F-A♭-D♭-F). The last three notes are D♭-C-B which is a common melodic figure over a N6-V transition.
I'd say yes: Beethoven is kind of on the nose here and outlines a D flat major chord in the 5th bar of the excerpt. It's followed by dominant-function leading tones. Right after that and outside of the excerpt, G's play, and then the rest of the piece continues with C minor chord figurations. Sounds like a Neapolitan chord that properly resolves to me.
Interpretation ambiguity can still reign, though: the 3rd and 4th bars of that excerpt can easily be interpreted as outlining an F minor chord, IMO, as long as you ditch the E in the third bar early in.