A couple of general answers:
Anything pertaining to encouraging or discouraging certain kinds of migration will be dealt with via the as-yet-unfinished Migration Policy Laws.
When I mentioned a cultural minority might "demand self-governance" under certain conditions I'm referring to the Secession mechanics, which we will talk about in the future.
There is no direct interaction with Pops in Victoria 3, all interaction are through other mediums like Buildings, Interest Groups, Decrees, Institutions etcetera. Similarly there are no direct interactions with cultures or religions, for example you're not able to ban or encourage migration of Pops of a specific culture. All such interaction is handled through the Citizenship and Church & State Laws, which determine if a Pop is Discriminated due to culture and/or religion. It's the distinction between non-Discriminated and Discriminated Pops that determine how specific policies work, rather than selecting a specific policy for a specific kind of person. Not denying that happened on occasion, but it's certainly less common and not the level of granularity we want the game to operate on.
We've considered and experimented with adjacency-based migration in the past, but ended up deciding not to have 3 distinct systems for migration in the game. Economic migration (poor Pops moving to where the jobs are) is handled via market migration. Turmoil-based migration (destitute or oppressed Pops moving abroad) is handled via mass migration. While cross-border migration as well as a steady trickle of global migrants are certainly not ahistorical, it is irrelevant for a player when the effect is very low and difficult to explain or do something about when it's already very high. Economic migration is usually something a player is happy for and can control to some degree with Decrees, while Mass Migration is easier to signal and explain to the player and can be controlled with Laws. The effects of other forms of migration are also less relevant to model and would result in a lot of Pop fracturing (and performance problems) for no major discernible benefit.