Get your various stories straight.
Crawford did say in the '60s that Dunaway was "the only actress with enough guts today to be a star" (and if you watch Joan in her early films like GRAND HOTEL you can see the Dunaway clearly in her, so there was a recognition factor going on) and, allegedly, she said Faye was the only actress who could ever play her although that quote has been disputed.
Faye Dunaway
had
to have heard about that. So if she was so moved to later say that "Joan's spirit isn't still" over MOMMIE DEAREST and also claim that Christina was "a child of privilege" (apparently there's no child abuse amongst the rich and famous, or if there is, you should shut up if you lived in a mansion) then why indeed did Dunaway lobby for the part after Anne Bancroft left the project??
The 1978 book of "Mommie Dearest" was very famous at the time. Everyone knew what it was about. So if Faye thought Joan was being mistreated by the treatise, why do the movie??
BTW: Christina's brother confirmed the abuse by Joan, as have others; their two younger adopted sisters, who weren't raised with them, occasionally denied it (and one of their kids, who wasn't there, is trying to "clear" Joan's name).
LBJ's mistress on JFK: