I'm a librarian who loves anime, manga, and reading a wide variety of genres.
"True, you've got some...character flaws...but you're not a stalker. I think it's a possessive urge that drives stalking behavior. But you put your life on the line for Yagiri-kun's sake. I don't think you could do something like that solely out of selfish desire, right?"
I really hope Mikado is just lying to try to smooth things over, because she is most definitely a stalker. Seiji sat next to her once and did something nice for her once. Based on those two things, she chased after him relentlessly, and even invaded his home. Saving his life doesn't erase that.
Just like in Baccano!, all the canon romances in this series are seriously screwed up. I loved Baccano! despite that, but for some reason I'm more iffy about this series.