It seems to me our concepts are quite similar.
My question is, who do you envision creating the laws? (Practically, in the game.) There are various possibilities, perhaps not entirely mutually exclusive.
Of course, the founding civilization could have its own fixed set of rules, but taken to extremes, that seems rather dull. You would essentially be stuck with a single-faith fort from the beginning to the fun. (Although this opens some possibilities for the patriotic rules and traditions you mentioned.)
The second basic possibility is for the fortress to make its own laws, which could either happen automatically (once a religion achieves a certain position of prominence, it's promoted to a certain status and starts issuing binding rules in a way analogous to nobles) or it could be done by players decision - the dwarfs would demand and require some rules and become upset unless the player yielded to their requests, but it would ultimately be up to the player to make the decision.
I think the fortress is, basically, a "colony" -- its relationship with mother government would be defined mainly by the distance, measured in how long does a journey there take and how risky it is.
If the fortress is close, it should keep close contacts and get bigger migration waves from the motherland -- migration waves to far places might be thinned by dangerous terrain, animals, plants and even some of the nastier minerals before they get to you.
Migration: that's the key. Your seven starting dwarves are fairly monolithic. They come from the same society, they will likely share the same values. Migrants are not limited to your dwarven civilization and can bring multiculturalism, new ideas and new problems.
Looking at the migrations in the real world, there are several types:
1. Official migrations -- government sends dwarves to settle in your fortress. All would come from your civilization and the size would be dependent on distance, as they could die on the way. These would be generally good citizens of your motherland.
2. Penal migrations -- criminals exiled to your fortress. People deemed "unsuitable" for your civilization. These would bring new ideas, they could be heretical cult leaders, etc.
3. Immigrants -- coming from any civilization.
And you could eventually either become the capital of your civilization, or declare independence