My thoughts on why Chrissy didn't use the super idol..
apparently her and Katrina weren't as close as it made them seem on the show with the edit they got.. however, without even considering that..
There was 1 vote for Ashley and the rest were for Katrina. Even Chrissy voted to get rid of Katrina. If Katrina was the only person voting Ashley, Ashley would have went home.
This would have caused conflict with JP and because she kept the super idol secret, it would put an even bigger target on her back. Essentially, she would have been going against the whole tribes wishes. Although it would solidify an alliance with Katrina, the 3 guys would go after the girls next. Alan was trying to break up JP and Ashley but since she was voted out, he would just stick with the guys.
Chrissy would be the target though after pulling the idol stunt and Katrina would be spared the next time they go to tribal.
Just my thoughts