Still, Alissa is still technically Queen on paper and through real history we can see many hated Queen and Kings getting reinstated. And as long as it is not officially recorded, anything could change. Hell, even if it is official as you could get, that never stopped humanity of pushing the laws under the rug in vain attempt for selfish gain.Only because the laws are apparently written in such a way that she can't be forcibly stripped of her title, only by judicial combat, abdication or death, and neither Ryn nor Elthara thinks that ignoring the laws would be a good idea. Alissa is however explicitly declared an outlaw subject to arrest if she ever sets foot in the city again and since all her toadies are dead or corrupted and pretty much the entire city saw her become a demon, there's nobody who particularly wants her back. That Ryn is regent rather than queen at this point is more or less a formality.
Savin has talked about how this can play out and low-Confidence Ryn can by implication become a figurehead while the Champ exercises the real power if you want to be all corrupt and shit, but just because 'Corruption' is in the title doesn't mean that every possible way for the player to be a colossal jackass is going to be in the game. Ryn's complicated enough without writing an entire 'and then I put her evil bitch of a sister back on the throne for shits and giggles' gamestate that would have to be accounted for as well. Which you don't have the authority to do anyways.
Now, considering what Elthara said about the whole "lack of nobility" and the fact that we can become one, getting to that point seems feasible since reinstating a Noble in old times was as a daily yearly expirience as you could get. Especially in times of great turmoil.
Especially if something happened to Ryn (like if she died or something along those lines) since than Ali would remain only remaining loyalty you know.