It's not that he was very confident. He knew from the beginning of the story, ark 2 during the school competition, that he would have no problem beating him because his magic was a perfect counter to his. Tatsuya, the first second that met him, both manga and novel said: "I need to find a way to...
If she has bones as strong as she says. That's a genetic abnormality., some people are born with bones twice as thick and strong as normal humans. Their bones never break, in fact the skin would tear before the bones break. The problem? You cant swim because your body is so heavy that you go under.
When she died, she didnt cross to the other side. She stayed and saw her family fall apart after her passing. That's why she was able to possess marika body. Last time, she was at ease. So the possession did end as much as her soul started crossing. It came back after the body saw the marriage...
The only good piece of development in this chapter was the beginning dialogue. With saijou turn been over. I'm guessing she is referring to that he already played all his cards and she still refused all advancements even when is clear she likes him. Now its natsukawa turn to show actual care for...
I believe he also drinks the same potions as jinshi to avoid... been discovered. When jinshi was drinking it, he advised to stop. As he might be sterile if he keeps going. Jinshi then says to him that if he is not the same, he then answered that he already has children. It doesnt matter