So I guess that the remaining human survivors formed a nation, but what form of government exists in this nation? Who is the leader?
Seeing as there are mentions of a royal family being inside Wall Sina, it's safe to assume that the government is monarchical in nature. However, I am leaning towards a monarchical-parliamentary style, rather than just pure monarchical or parliament. Reason being, even with hints at a king being the head, the actions alone of the various commanders belie an amazing level of independence. In addition, it's been seen a couple of times that they can be summoned and tried in a public court, a trait that is definitely not purely monarchical in nature (as kings alone judge in that event). The voting system to approve a course of action mirrors a parliamentary set-up as well.
It is monarchy because they are protecting not only the citizens (by its level) but also the current king of each designated wall. You can see it as Commander Pixis is used to play with the king but generally leave after he heard the attacked of titans in their region.