Start at a later date when Venice has more land and you can get a successful invasion more easily. As I recall there's a point where they own part of Carinithia, part of Dalmatia, Crete and a few other counties in Greece. With that in mind you can have two whole duchies in Italy and spend time getting in the Pope's good books so you can get the invasion. I can't think of any other way to acquire it, because the usurp screen once it's become de jure says "Government must be "Republic""
Genoa is a duchy title, and will always be that way. That's why you can usurp it and take it over as feudal. I don't know of any duchies in game that have special creation conditions, probably because counties can't de jure drift out of them into other duchies. Kingdoms are different, as the whole idea of Venice is that it gradually departs from feudal Italy and instead becomes a legally seperate republican state. If you want to govern Venice de jure after its drifted out, you're supposed to drift it back in to the old feudal system it was under.
