[ChrisMAg2]

ad 1: Yes, but
Fahnenjunker equals an "Unteroffizier". Actually he is the one responsibel for the flag, and the actual bearer.
Fähnrich equals a "Feldwebel" or "Wachtmeister"
Oberfähnrich equals an "Oberfeldwebel" or a "Stabsfeldwebel".

ad 2:
He could start even lower:
Soldat/ Unteroffiziersanwarter- Fahnenjunker- Fähnrich- Oberfähnrich- Leutnant.
And to make it a little more difficult, you don't have to take all steps. It depends on a lot of circumstances.

ad 3:
I don't know if it is correct to directely compare the different rank systems by name. I was told this shouldn't be done because the different ranking systems are really different to each other.

Stabsfeldwebel: Simple: It's one rank higher then the Hauptfeldwebel. But this simplification is more then crude. The different ranks are coupled to the assignment (Tätigkeit und/ oder Funktion in German), or specialisation, not so much on the carreer or promotion.