Except that Stanley wasn't popped as an heir. He was promoted to heir. There is a big difference. After that distinction, the rest of your argument falls apart.
...and London is the capital of England even if HM the Queen travels abroad. We recognize a capital regardless of whether or not the ruler is currently there or not. Why must Erfworld be different? Erfworld is not just a game. In fact, it may not be a game at all. It may just be a world that resembles a game.
I actually do draw a distinction. I think that 'King' and 'Queen' are titles reserved for overlords that happen to be royals. Just like I think that royals are nobles that are overlords or heirs. Creates a nice little hierarchy. No proof, but it makes more sense to me than 3 unrelated categories that just happen to overlap slightly in function/role.
Actually, I feel this is justified by the comic. The very fact that you can promote a unit to heir and not just pop an heir. If it you had to just deal with the heir you've got, why would the titan's create an alternative method to have an heir?