Very good points. The structure is still essentially a feudal society, so its a very vertical hierarchy. Within a Family's territory their Sovereignty would generally rule, and could only be overridden by somebody higher up the ladder. So things like taxation, legislative powers, functionary appointments, etc would be handled by the local Noble, so long as it doesn't contradict anything declared by a higher up (as each essentially rules in the name of their boss). And only a superior could countermand them, though such Interventions are generally avoided, as there is a rebuke in it, stated or implied. If you need to go past that, you need to get a sponsor that is higher up the Food Chain, but that would be like somebody in France getting England to complain to the UN; Most would rather not get in the middle of internal disputes, and doing so is generally frowned upon. i figure it would have to be criminal charges (Violations of Charter Law, ie human rights, treason, etc) or something equally important. Each Major house will have widely varied cultures and styles, as much as any foreign country (single language though)
Offhand, the rough hierarchy would be:
1. Serf - houseless peasant
2. Family - a traditional extended family, usually in a particular business (Tailor, shipwrights, farmer, etc)
3. Merchant Family - Wealthy, Landowners. The banks/Moneylenders/corporations (though probably not literally). Basically a collection of the most influential Families of the City.
4. Noble House - Local Government
5. Major House - State Government, Nobles are all bound to a Major House (geographically contiguous territory), and a Capital City where they also responsible for the Local government
6. Throne - Responsible for a central Capital city, as well as some territory of Nobles equal to a Major House (thus whichever Major House sits on the throne controls something like double the territory of the others... perhaps its geographically limited, like a large Island capital and several smaller island Nobles)
NOTE - there may be more layers between the city level Noble and the Major house, but it would follow the same pattern, just increasingly larger territories.
There will be a Charter that includes the Rights and Responsibilities of each level, including basic human rights, as well as outlining the various Trials. A mechanism to challenge the local boss would be needed, so maybe everyone is allowed to go one step above their boss. How about: Anyone has the right to petition the local Noble House for an Intervention, though a serf would have difficulty being taken seriously without a family to sponsor them. A powerful Merchant Family could go past the Noble to its Major House overseer if the matter is dire enough that it would impact the liege. A Noble House Could petition the King to intercede against their Major House bosses. The Major Houses would have a Council of their leaders, as a balance to the King who has no Boss. I dont think they should be able to override him on just anything, or else his power would quickly erode. I figure the most they can do by Law is some sort of Impeachment, a vote of No Confidence for extreme circumstances, which would require an equally extreme majority (maybe even a vote of the Lesser Nobles as well, or support from the Guild Council or something. Or maybe there is no Law based recourse to depose a bad king, leaving assassinations or some secret Failsafe (magic, secret society, etc). That being said I would see the average King delegating a number of things to the council, or seeking their advice as advisers, but the at the end of the day the King's Word is Law.
Although I am considering having a Trial by Combat option, whereas you can challenge somebody for their position by proving that you are better fit to defend the Land. It wouldn't be the only trial for leadership, but it would be one of them required for the throne. A king would have to be able to defeat a Master of one of the Circles (ie a Guild leader or equivalent) or maybe all of them, to prove himself the most powerful in the land. Hmmm, one trial per Circle would make sense. At least one but not all would be combat, though all would likely be potentially deadly; you cant hold back when choosing a King. For lesser positions I figure theyd need to pass a certain number of trials (more trials for higher levels) to the satisfaction of a certain number of their future contemporaries, or a smaller number of a level up, and so on.
Actually changing the Charter would be a very rare and difficult thing, as much of the framework will be symbolically tied up with the Astral Theology. It will come up sometime in Book Three, but Im thinking that will be something unprecedented in their (current) society. A big part of the MC is that he has magic that does not conform to any of the existing Schools, and breaks many of the established "rules" of magic. His birth was also during a particularly rare astronomical event, so he was raised under a certain bit of stigma, expectations of Greatness beyond even the Throne, and so facing jealousies and such since birth, and perhaps even some radicals thinking he will destroy teh world or some such catastrophe.
I agree that fear of invasion will bind a country together. But I wonder if you overstate the need to bind together from fear of invasion. I don't think the USA much feared invasion after 1825 or so. (We were too big and Europe was still recovering from the Napoleonic wars, IIRC) Tho we did have ideas like manifest destiny to bind us together. (And slavery to tear us apart). Perhaps nationalism would do the trick?
Poland of the late 17th and the 18th C comes to mind. Prussia and Russia started working on the magnate's sympathies, offering titles and other inducements to gum up the works of the Polish Seym (congress). They kept the king too weak for too long and when a strong king finally started making progress at reform, Poland's neighbors were strong enough to just partition the kingdom. Note the fear of foreign invasion was not enough to keep the magnates aligned with Poland's interests, since foreign lobbyists convinced the magnates otherwise.
True we didnt have a heavy outside enemy, and so we started drawing lines in our own country and ended up in full civil war within a few decades. But you raise a good point. An outside force that is an active political force will be more divisive than not. The Outside Enemy really needs to be a Boogie Man, something that nobody would work with, everybody fears sufficiently, and ideally nobody identifies with. A Language barrier would need to be a minimum, but im leaning towards something more monstrous and/or less accessible. An evil from the past (monster horde, barbarians, aliens!), rather than a neighbor. Id rather keep the politics internal, with the different Major Houses playing the role of Neighboring governments and foreign cultures. The Enemy would need to be something bigger, nastier, something no sane person would work with, if they could communicate at all. And something that would warrant the upheaval that the Prince is basically prophesied to bring about.
BTW can a crown princess become the sovereign or is it limited to males?
Magic won't favor either gender over the other, so I dont see the main social order doing so, certainly not for choosing the King, which would be outlined by the Charter. Any candidate put forward that can pass the Trials would have proven themselves worthy, regardless of gender. In general the line of Succession would go by birth, then to adopted Family (unless politics push otherwise), which would be given an ill-defined rank of Cousin in the genealogy. But that is just the Order by which they can attempt the Trials. If the person in front of the line Fails (which may or may not be lethal) then the next one in line is free to step up and try.
I could see there being House the favored one gender over the other. Actually I like that idea, a Matriarchal House that tracks bloodlines through the mother's side only, and only ever puts forward Female Candidates. They'd be the more oppressive, narrow minded house. Thats good. I needed an antagonistic contemporary from another house, but the male was becoming too Draco Malfoy for my tastes. A more Conniving Bitch character would fit better.