Clearly Carroll is not happy with Nagasu- perhaps the two should part ways. Nagasu may need a coach that is more supportive and willing to encourage her through rough times- but is also not afraid to slap the hammer down when necessary. To me, it sounds like Carroll does the latter and not enough of the former.
OR- maybe Carroll is right, and is telling it like it is. In which case, this might indeed be the slap that Nagasu needs to pull it together. Sometimes it really takes a major disappointment to evaluate where you are and make the necessary adjustments. Missing the world team should do it.
The problem with Nagasu that night was not the mistakes...IMO, it's that she gave up. She was nervous...fine, but once she made a mistake that was it. You just cannot give up on a performance like that. With Cop it's not over if you fall. You can still make up points here and there. THAT is why she was not on the world team. Flatt's performance was very beatable.
ETA: I agree with Hersh that we probably need that third ladies spot more than ever now. It enables an additional skater to gain Worlds experience. With our field, that could be invaluable. Come on Alyssa and Rachel!
To me the issue is not whether Frank should tell it like it is. The issue is whether he really needed to do it so publicly instead of just to her.