The reason these 2 do it is because they shared their first kiss with Rentaro, so their love for him is also kinda connected to each other.
There is also a theory that, since they were all kind of looking at each other when the first thing happened, that this might be an actual three-way...
No, they already found him so they wouldn't die. They would die if they didn't end up together, so basically every other girl we haven't seen yet probably would.
She is pretty close to an idea I had for a girlfriend. I had the idea of a yandere that was obsessed with the whole group and knew everything about them, basically being a 2nd Rentaro. But unfortunately, she isn't a yandere, she's just lonely.
The only things I can think of is occasionally bringing up that her body gets much hotter and the fact that everything she needs has to be made by the government but that department kind of waves away a lot of issues.