Clark stops the cold guy at the end by thowing him into a lake or pond, which almost immediately freezes solid and apparently traps him. I'm wondering why the lake he fell into at the beginning of the episode when he gained his kryptonite power didn't freeze solid on him in the same way.

Because he only needed a weak campfire in the beginning to sate him, and a nuclear generator at the end. My guess, anyway.

What I don't get is where Sean was when Whitney swerved off the road to miss him. Surely Sean wasn't afraid of Whitney, so why didn't he go after the jock and Lana? He was in the darn area!