I agree that her plan doesn't have putting the MC on the throne at the heart of it.
Well, the king being weak in body, mind and rule is the reason she states for the coup, that he can't damage the country any further.
But yeah, i don't believe that is her real reason.
True, there is zero reason bring in other powers if the goal is to bring the MC on the throne.
Why even go with the plan to isolate the king and let the MC play regent ? It's risky and if discovered treason.
Cass clearly has no problem with killing, she clearly hates her father, she has the skill and plenty of opportunity. Add to that his health is really bad, so none would suspicious if he would die in his sleep. Or a poison, don't even need to kill him, in his bad health the right poison would really incapacitate him. Even if other nobels would get suspicious just show them the king and all is good, barely any risk involved.
Then there is the things of what Cass wants from the MC, like take her with and not kneeling, ridicule the MC at the dance, don't follow traditions or the order, be rebelious etc. also not being heir...if you take her plan into account that is the opposite of what you'd want.
A MC which follows traditions, respects his father and is named heir would be far less suspicious and way better for the plan. As you said, why would he try to kill his father if he already is heir and his father's health is ailing.
I mean, call me paranoid, but if you do what Cass wants it looks more like she tries to make the MC look as bad and unfit to rule as possible...
Yes absolutely, the last thing you'd want as king is giving all the power to one person. Even less if that person is a spy/assassin, one who wants to be queen, is putting her allies in all key positions and clearly plays her own game.
Yes aggred, i think she genuinely likes the MC.
But i don't think that would stop her from making use of him, using him as puppet, or try to take the crown for herself. I don't think she would kill him thought, as opposed to Isis, which clearly would when the MC has served his purpose.