Um... there were literally no hints that mikado is interested in Kaguya... this came out of left field.
The first time Mikado spoke to Kaguya when he transferred in, he called her "Princess" and spoke like they knew each other already. Even after she said she doesn't like that name anymore, he kept using it, meaning the name likely has some importance to him. I agree that Mikado was introduced way too late in the story, but there WERE clues he has something to do with Kaguya from his first real introduction (I'm not counting his sister's India trip as his introduction).
Also, I was right in assuming that Mikado is the intended marriage partner and he doesn't really want to marry her, but I was wrong in assuming that he had a plan to sabotage it. Him just expecting Miyuki to do something isn't really a plan.