Dear folks, it's not enough and it's never going to be enough, but the author finally gave clarity in iff vol. 28-29
Vol. 28, a classmate asked Touma if Mizuhara and him are dating. Contrary to his vague answer in Q.E.D vol. 16, this time he said "It's not official yet, though."
He also mentioned it to Mizuhara nonchalantly.
Then in vol. 29, he asked Mizuhara to go out with him once she gets accepted to Harvard and everything is settled down.
He asked it very lightly, although still in serious manner. Mizuhara, of course, answered "Okay" immediately much to Touma surprised.
He even asked if it's alright for her to decide it that quick.
She confirmed that she doesn't have any doubt because it feels right to her.