Forecasting timmeeee, my predictions:
a) Kinka won't be able to die now. Probably because Kazura throws herself on the train track and dies with her.
b) They won't be able to die ever, because Kazura first is in love with Otome, but will then be in love with Kinka.
c) Kazura will fall in love first (duh).
d) Kinka's motive is work related but quite probably not social. If it involves another person at all, it is either romantic (based on page 140, chap 4), and then probably a woman, or family-related. But I think these two topics, love and family, have already been explored with Kazura, so I'll stick with my prediction that it's non-social.
e) Yamayashi (Kinka's work friend) is not an important character and, if we see here again, it will be for a single conversation and probably because Kazura initiates it.
f) Otome was unhappy and angry when she got her first boyfriend because... she wanted to pressure him to buy her all kinds of stuff. But he didn't cave in. So she tried the same thing with Kazura, who accepted it and that's why Otome says she might even fall in love with her (because she's willing to do what her boyfriend wouldn't).