Thanks for answers. Concerning this:
How quick is that spread and is it enough to have 0% effective revolt risk or does the existence of any negative modifier, even if off-set by higher positive modifiers, enough to stop the spread?
http://www.paradoxian.org/eu3wiki/Culture#Cultural_Assimilation_Event
Things like Narrowminded vs Innovative, Free Subject vs Serfdom, size of the province, etc, are all taken into account. Theres a whole list of modifiers and triggers there. I don't think it changed much in DW.