This post is mostly about world-building in response to Durakken, so if that bores you, don't bother.
Durakken, yes, as I said it, it's a weird mix of both concept map (their POV, my detailed needs), I never said it makes sense! But it works for my world-building purposes. I only posted that political map as a nod.
You add very interesting stuff. The Roman Empire along with the Persian one are two of the main inspirations for mine, but the actual location of the Empire has changed quite a bit with each revision. Its current position came from adding a layer in PS with an average temperature map of our world and adjusted to match the latitudes in my world, the placed the empire on the continent I wanted yes, but put the limits where the temperatures worked best for me.
Picsi and Hemlia, and the other power's approximate borders, are known to the Empire only through maps they got from the other powers by shady means, the Empire doesn't have detailed maps about them.
Arlians have advanced naval tech, both for trade and war, at the level of the Late Middle Ages let's say, so it's not so weird for me to think they'd be able to get to Picsi through the islands' chain.
Doesn't seem that farfetched for me that looking for new markets (and following rumors about new lands) the Arlians or Oncar would find Hemlia, and from there they could follow the islands on the west to Enenlia, or would find the little islands first and then get to the big lands.
Arlians got to Arlia escaping the advance of the Empire, a bit like Pilgrims that got to the Americas from England, they ended up there because of a series of reasons and mixed a bit with the natives. So they have common ethnic roots and language to the population of the area where the Empire grew from.
Arlia, Picsi and Hemlia have nothing to do with each other except trade. The light colors on the map are areas of trade influence, a bit like the Greeks and Phoenicians had going on the Peninsula Iberica before the Carthaginians and Romans came to stay.
About the capital for Swifendlia: what the 6 powers do is act as trade re-distributors among themselves, since each guards their own areas of influence so the others don't mingle on it. It took time and lots of small conflicts to get to the current frail equilibrium.
So not much is going on on the peninsula on the external side, since trade between Arlia and the Empire can take place near the borders of their areas of influence.
Inside the empire, I don't find the peninsula specially significant for trade (yes for defense, since it had narrow land borders and has a bit of height), since it would only be another stop on the route. I mean, they'd be enough cities along the coast on the Esilec sea, so I don't really see what a city on the peninsula's coast would be more automatically significant than the others. What am I missing?
Plus the capital stayed there through the centuries on purpose, for political reasons, since the ruling dynasty originated from around that area and they believe they're the chosen ones and that crap, so of course they'd set their capital on the sacred land where they came from.
I agree about moving Ontir to the west to be on the mouth of the entry to Toec, near one of the two small rivers around there.
Yes, Inalia is weakest of the 3 and her navy, as the Empire's, is mostly just good for coastal navigation. Oncar probably should control the area of influence between itself and Inalia. I figure the Empire didn't want Oncar near in both of its ends (since Oncar and Arlia and the only 2 powers that can be potentially problematic), so they favored having Inalia in the middle on the North as a buffer and would have had a hand in stopping Oncar from advancing further.
I have not figured the details about 4 of the powers and their dynamic to be honest, I'm just warming up to them existing at the moment, since I had only conceived the Empire and Arlia until I did the last map.
That said, I loved how you made me have to think about it all Durakken. I don't agree with all that you suggested, but perhaps only because you didn't add much detail in your arguments to be convinced.
From how self-assured you sounded it feels you've got experience in this sort of thing, I guess from working on your Web novel, where did you get your guidelines from? My arguments sound kind of weak against yours. I can't see any map of yours, the links on your 2012 thread are dead. If it's a project you are posting about somewhere I'd love to keep an eye on it.