It seems to be impossible to spawn Colonialism institution in France - requirements include 'must have a port in Paris', or words to that effect. Is this right, or am I missing something? This would also seem to make it impossible for Castile, or any country whose capital isn't a port.
It spawned in France for me a few times (Castile too), including the reload I went with*, so it is definitely possible.
*I'd been reloading, trying to get it to spawn in my capital (custom nation in Cape Verde) before I came to the understanding institutions never spawn on islands.
You are missing something. Specifically, you are missing the fact that the interface for telling you how an institution can be spawned is a pile of crap in dire need of upgrading.
Any province that satisfies the criteria described is a valid spawn point, but the user interface will only tell you whether your capital is eligible.
Yeah, that tooltip did not help me either in the aforementoned regard.