Oh, poor Rumia...

Let's see if I'm getting this straight: Reimu is thinking about using wrestling movements against Rumia while she thinks she is going to get tickled, is that it? Althought Rumia's expression seems to me as if she actually wanted to be tickled.

No, it's that anticipation you get when you know it's happening. Like how when someone goes to tickle you, and you just get that feeling and react before you even get touched.

Rumia thinks Reimu is playing with her, like good friends.

Reimu was trying to do wrestling movements, but since Rumia is enjoying it, is a failure.