@MarqFJA87
I would think he'd be aware of that because of series like the Fate/series showing "Kings" who are biologically female, but shown to the public as male. But maybe Takitarou doesn't know that the title of King is just simply a title, and that females could ascend the throne as King if they are the only legitimate heir.
Pg.07 says "Oshida wo hikiai ni daseba". I believe she's saying if she can get Oshida to go along with this, she'll be able to go through with no problems. I think Marie's worried that Oshida may be so repulsed of Andou being Commander, that she won't stand for it and make it harder for Marie to enjoy her now carefree life.