As a half sicilian myself (on my father's side), I can at least answer the questions about the sicilian language, which actually is an italian dialect. BTW, I speak italian fluently and I understand and speak a little sicilian (seeing my relatives in Siciliy almost every year).

The sicilian dialect is significantly different from, let's say "clean" italian. It contains a lot of words deriving from spanish and french, even some arabic. An italian from the northern part would hardly understand a word.

However, not all italian actors in the movie speak sicilian. It's basically some kinda southern coloured italian with a few sicilian expressions. The beginning of Part 2 is a good example for real sicilian, though (even if it sounds a little "learned" at parts). For example they tell young vito "Nun ti scandare" for "Don't be afraid", instead of (italian) "Non avere paura". Or "picciliddru" instead of "bambino" (kid). This should show you how different the two languages are!

There are even sub-dialects in the different sicilian provinces (towns). They, however, differ from each other mostly in the pronounciation (melody).

I hope I could help!

Greets

Don Lanzarone