Yes, there's a possibility that you will get no troops when you update buildings, which is why the recruitment viewer is so crucial.

I don't believe there's much of a difference between militia and local levies.

So, you can build native barracks and regional barracks. You can only upgrade native barracks to its fullest if you installed a level 1 government, and regional barracks if you installed a level 4 government. The further you are away from your homeland provinces, the less of a chance you'll get troops from native barracks and more of a chance you'll get troops from regional ones. So it is usually wise to not bother building native barracks in those provinces far away from your homeland areas.

As for the recruitment viewer, I like using the province viewer or whatever it's called. It's one of the things you can choose to use if you look through the drop downs in the upper left. You'll find it. Just open that up, pick a faction, pick regional barracks, native barracks, or both on the main screen, then start selecting provinces. Make sure to click on that checkbox that allows you to see the data so you know at what barracks level each unit is available at.