Just a few points I've found playing and tinkering...
1-Indeed, some countries cannot arise past (or pre) certain dates as specified in revolt.txt.
2-I have seen new countries (such as Transylvania) that were not in existence in 1492 arise so brand new countries can come up (Netherlands is special as its own super-strong nationalism % comes to play post the Calvin event, and that is hard-coded in the game so NET almost always gets born).
3-Scotland does have nationalism values, however Scotland is part of the national sphere of England so an English conquest will not trigger nationalism (if you want, just take off Scottish provinces away from england in the .inc files to make SCO rebel, maybe leaving Grampian in the English sphere to leave England with a nearly-permanent casus belli on Scotland).
4-A new country has non-zero odds of forming provided its capital and any other must Province (as per revolt.txt) has rebelled. Revolt.txt lists further optional other Provinces that will join the new country upon formation if they too are rebelling. When the new country has more than one Province, these Provinces can be taken at the expense of any number of countries but the declaration of war against the former owner only occurs against the former owner of the new country's capital. Example: FRA owns Flanders, SPA the rest of the Low countries. When NET pops up, Flanders joins the new country but FRA and NET do not go to war automatically, only NET vs SPA.
5-New countries stand a higher chance of arising (and of being their full size) when a government falls or when civil war is declared. Quantitatively how much I do not know.
Thx