It can be confusing at first to figure out harmonies when there are only two lines and the chords are not filled out. The best approach is to look at the bass line to figure out what is going on. Your chord in question resolves to a unison F# in the first beat of the next measure. F# minor is the relative minor of A major, the key of the piece. The bass note in your chord is a C# which moves to F# in the next chord. So your chord resolves to F# (and it's an implied F# minor, since we've not heard the A# anywhere that would be required to make it F# Major).Now, what chord naturally resolves to F# minor? Well the dominant of F# minor is C# major, and that is, in fact, your chord. Because there are only two lines in the texture, all you get are the Bass C# and the Soprano E#, while the fifth of that chord, the G# is not there. Still, it's clear that the implied harmony is C# major to F# minor.Also, I would not call the chord you marked previous to the blue one V/vii; it sounds like G# minor, which would be the ii chord in F# minor. So if you think of the measure you are interested in, you can imagine it in F# minor, and the progression is ii-V-i (G# minor -C# major - F# minor) a very common cadence in F# minor. The piece shifts back to A major shortly after that, but the above is how I'd think of the measures you asked about. The rest of your analysis looks correct to me. Nice job.Hope that helps.
It might help you if you view the entire stretch bars 17-24 as being in f sharp minor and analyze with f sharp minor as the tonic in mind.