You know, I'm honestly surprised that the Three Sages are actually legitimately working in the best interest of the Empire and have seemingly no interests in taking it over. While they treat Stella more like a political tool and public face, it's actually very fair if their goal is to avoid war so that's surprising that none of them are thinking of trying to profit off of war or increase taxes and skim it or some shit. It also seems like they balance each other out with their opinions and view points too with one being from a knight family, one a public official and the last a merchant perhaps. Actually competent and upstanding administrators in an isekai?!?
To me it seems a bit off because they aren't considering Stella's importance as a figurehead.
If I were one of them I'd evaluate the current line of succession as follows:
1. Stella: legitimate claim to the throne, presentable, not particularly assertive. Overall verdict: Excellent puppet ruler material, won't interfere with us ruling the country.
2. The two stepkids: dubious claim to the throne, willing to hire assassins, clearly power-hungry. Overall verdict: Bad puppet ruler material, high chance of them trying to take away our power or replace us.
3. Some rando: would almost certainly replace us with their own people. By the by, the rando can't be me because I'm probably evenly matched with the other two guys and at least some of our power comes from the current royal family: none of us could take over the country on our own.
Basically Stella is by far the best option for me. The only potential issue is if she marries someone who's hard to control, like idk a war hungry prince.