When I posted the thread, I immediately acknowledged how awful that movie was, however I believe she did a decent job with the horrible material she was given. Also, she was 19 at the time that the movie was filmed, and did all in her power to try to get out of the obligation of doing the movie. She has never kept it a secret that she never wanted to do it.
FishermanBob: I have been a fan of Clarkson's for years and I have always thought she would do a good job in Theatre. If any of the people you listed could handle performing 8x a week, it's Kelly Clarkson.
LIza's Headband and AnthVoice2010: Not sure when you saw her, and obviously you're entitled to your opinions, but I saw her on the "All I Ever Wanted" (2009) and "Stronger" (2012) tours and she was anything but boring. She was engaging, funny, and of course she sounded amazing.
Not that she's right for Sandy in any way, but here's a clip of her singing "Hopelessly Devoted to You" from Grease. I don't know how many people on here buy her music or listen to anything but what you hear on the radio, but she is capable of much more than just screaming her face off on a "Stronger" or "People Like Us."
She starts around 1:30