The Etharch rules over everyone. That seems clear. The Lord Protector is commander of the military, although the Etharch remains commander-in-chief.
The Althing rules over a citadel. Although the name drives me nuts; the Althing is the Icelandic parliament. I’d make it atheling, prince, but the prince role is taken. I’m not sure whether Althings are members of a ruling body or if each one individually reigns over a citadel.
The role of princes seems a little odd to me. They’re ruling princes—but ruling over what, exactly? Ruling at the Etharch’s right hand as a very important aide? Some kind of superior over the Althing, or even multiple citadels?
The one thing I really have to insist on is axing the “destined to one day be Etharch” line. Because there are Fea princes who, well, aren’t.
The way I’ve generally put it together is that the Elves have something of a constitutional monarchy. The Althings are the elected ruling council, but they often answer to a prince. Relations can be good or strained. Some princes come from princely lines, so on the death of a prince a son or daughter, also a prince or princess, succeeds. The Etharch’s line are also princes, and may also rule territory in their own right, but only they can actually succeed the Etharch.
In human terms, the Althings are elected or selected representatives of the citadel, a Fea but not Aman prince is somewhat analogous to the high nobility of Men (think dukes or, well, many princes not of the blood) and a Fea and Aman prince is like a prince of the blood.
Of course Elves don’t necessarily have human-like social structures. It’s quite possible that the princes reign over allodial territory—that is, they don’t owe feudal homage to the Etharch—but that in great matters the Etharch is the titular ruler. So it might well be that the Althings are like the legislature under the executive office of a prince, but the princes are the national legislature under the executive office of an Etharch who holds different but not absolute power.