I was wondering if there is any harmonic reasoning and mechanism in play with the modulation from C major to Eb major in measure 103? The only "logic" that I can think of is that in previous measures (such as 97 and 101), we are temporarily in C minor which showed flashes of the Eb and Ab, notes found in the Eb major scale. Furthermore the melody that connects the end of 102 and 103 is only a half step rise (D to Eb) which makes the connection smoother, and that there is a shared note (G) between the Em/B chord in the second half of measure 102 and the C minor chord in 103. Is there a mechanism name to this modulation?
A key shift up a minor 3rd is very pleasing. It doesn't really fit into functional 'cycle of 5ths' reasoning, but at least it's validated by having a special name, 'chromatic mediant'. And yes, when you feel one coming on, preparing the ear with a i, iv or ♭VII chord is often useful.
This tune also illustrates how a strong melodic line helps a modulation. Wherever that upward scale at the end of bar 102 ended could have made a good new tonic.
♭III contains the dominant note. ♭VI contains the tonic note. We can therefore use these 'pivot notes' to excuse them. ♭VII includes two diatonic notes, which allows it entry to the ballpark I think. But looking for functional reasons to use these chords is generally futile. ♭VII CAN act as IV of IV, but often doesn't.