Rest assured, there is a reason she joined so easily, and it’s related to the overall story arc. I feel you should continue reading so that you can find out the reason for yourself, but if you honestly can’t face it any more, I can just tell you what happens. Tiny hint:
The fact that she joins immediately after noticing Yuki in the room is important. Though perhaps not for the reason you might suspect.
Uh. Mostly uphill. Though the absolute lowest point doesn’t happen until the next book. Though at least at that point, Kyon actually grows enough guts to step in.