Tonic Prolongation

The reason I don't think the second chord is a Bb chord is because of the prominent A in the melody (which doesn't belong to the Bb triad). (I also just don't hear a Bb in the harmony.) As a Dm/F chord, however, it becomes a minor v chord (very rare and generally nonfunctional in Classical music, upon which this categorization system is based). One might understand this chord to be a substitution for the dominant (major V) harmony, however.

The reason I'm hesitant to call the E half dim. a tonic chord is because chords with 7ths and diminished chords of any sort are considered to by very unstable in the common practice period (and tonic chords, being chords of resolution, traditionally tend to be very stable). Of course, since this is a more modern piece, perhaps you could understand the chord to act like a tonic chord anyway (as a substitute for the minor vi triad).

Because the beginning and ending chords have so much in common (separated by a related but contrasting chord), one could definitely say that this portion of the phrase is based on a tonic-dominant-tonic model, as you pointed out. However, I think it's also important to take a look at the linear content of this excerpt. The bass moves from G to F to E in this section, with the melody echoing this stepwise descent (Bb-A-G). Because the second chord acts as a passing chord, it is better understood as an embellishing chord (rather than a structural chord). So basically, yes.

Yes, you are right. It is nonsense to ignore that 'linear movement of the base and the melody. I think in the end it has beome more clear to me what is happening and I guess that in the end, the most important thing is to give it a more or less logical place in your head. But really nice piece. And thanks for your response, I learn alot from this.