The standard modern textbook answer using basic harmonic theory would be to view it as a I-IV-V7(incomplete)-I sort of progression over a tonic pedal.
The assumption in most treatment of classical theory is that a I7 implies a dissonant seventh that will generally tend to resolve down by step. The seventh above the bass in this case resolves by ascending, so it's not a standard use of the "classical" seventh chord. So no, I wouldn't agree with a teacher who would label this as a major seventh chord, as least according to standard classical theory. (Jazz theory treats sevenths differently and less strictly.)
That said, there is at least something in the idea that the C-B is a kind of suspension: the C does want to resolve down (and ultimately does). Actually, it's not really a suspension, but a neighbor tone, a dissonant fourth above the bass. If you look at your reduction in the question, you can see an overall B-C-B motion in the upper voice, and a rising D-E-F♯-G ascending line in the middle voice.
That latter interpretation is closer to how Bach probably viewed this himself, as Roman numerals didn't exist yet in his time. He would have thought of this contrapuntally, with a static G bass, an ascending middle line, and a top voice with a neighbor tone motion. In effect, the B-C motion creates a dissonant fourth above the bass that needs resolution, then the middle voice rise to F♯ creates an F♯-C tritone that increases the need for resolution, which then moves to G-B in the next bar.
However that gets translated into Roman numerals depends a bit on what assumptions you're making (e.g., are we just speaking functionally and calling it a V7, even without the "root" D?). But yes, most modern theory texts would probably see this as a series of incomplete harmonies implying a I-IV-V7-I over a pedal.