Further analysis of Anna's claim re Derevenko suggests it's even less plausible. I haven't read it in some time, but...
Does she claim to have personally observed him berating Alexei? If she had , that would tend to explain why there is no other corroboration of her story. After all , Derevenko is soon out of the picture and in no position to rebut what she wrote. In fact I think her memoir may have appeared after his death.
That leaves Alexei himself. He lived about fifteen months more after the alleged mistreatment, in close confinement with his family and family sympathizers. What is the likelihood that , if Derevenko had mistreated him and shocked him thereby, that he wouldn't have confided this fact and how he felt about it, to his father,mother, Anastasia, OTM, Gilliard, or anyone else he felt comfortable with or close to? It would have been the natural and easy thing to do. But none of these intimates ever mentioned the alleged incident. This alone is very telling and make's Anna's assertion highly improbable, indeed, false.