If Nation A doesn't have main port range to Region 1, but they do have fort range then they can still siege the region.

If Nation B sieges region 2, then Nation C can still attend the region since Nation B was the one that started it.

While this does make sense, isn't the point of regions being obtainable by everyone to deter folks holding a part of the globe hostage in a sense? It works both ways for offensive and defensive but whoever owns the region's holds the power to siege or defend anyone within its SOI, making it harder to siege somebody when SOI already does that seems a bit much. If you make starting regions reliant on having a port/town in range, then battles over them will become less frequent as folks probably aren't going to make a port for a region if it doesnt already reach their nation, nor are they going to leap frogger forts until they can reach it.

i agree with hamjam, this limits nations and i dont see any real benefit from it. If a nation is strong enough to get a region across the whole globe they deserve that SOI in my opinion

it also gives nations close to regions a reason to fight for the region next to them to limit other nations soi that wouldnt otherwise have reach on them.