>Is it a dialect or a separate (although closely related) language to English?
As I see it, Scots and Standard English are two dialects of the same language.
>If these countries had not undergone unification in the 19th century would Duolingo be offering courses in Venetian and Bavarian?
Tuscan and High German became the standard with the Renaissance and the printing press, centuries before nationalism.
Different areas of Italy in particular had different literary vernaculars over time. One's that's particularly amusing to me is Franco-Venetian [1].