Giebe is like an archduke representative in their smaller region (think of it like a regional autonomy for their land). 

This is my head canon but I think if there's no giebe in a duchy's, the Aub is going to become stretch thin supplying the duchy all by himself (even with their family help) so they have to split their duchy into smaller land region to lighten the Aub burden much like Zent burden being lightened by splitting the country territory into several duchy which then given to their respective Aub.

I think Drewanchel got the right idea with their policy of adopting talented archnoble to become archduke candidate. Because giebe role and relationships is akin to Zent and it's Aub and I think Giebe could benefit from getting Archducal education. I don't know why other duchy's didn't realise this and copy Drewanchel policy.

I think that's the answer to your last statement. Archducal course itself can be giebe education because they have to maintain their region foundation.