wedding reception. If Kristina lies--she perjures herself. She'll be forced to tell the truth. If she tries to spin, Scott can get permission to treat her like a hostile witness which means he can ask her leading questions that require yes/no to his line of questioning.
Non-hostile LOQ: What did you witness between Sonny Corinthos and Dex Heller?
Hostile LOQ: Did you witness Sonny Corinthos violently beat Dex Heller?
The only things I can think of that Diane or Kristina can try to use against Ava are:
1. Releasing of the recording which isn't a crime.
2. Tampering with Morgan's meds, but there is no proof of that. IMO, Ava can lie about it without fear of perjury. If there was a smoking gun for that, Carly/Sonny would have used it as leverage long ago.
3. Killing Connie. Again, no proof or Ava would be in prison.