The Big Three ("B3" =Veronica/Elena/Yvette) state that Alexander's ultimate goal was a son from him and the perfect woman, or at the very least a son period, and the other sisters confirm this second goal. So why would Alexander destroy either of his lifelong goals is if Luke was his son, by Callista or otherwise. Veronica confirms in a B3 meeting that Kane is THE son of Callista (so Seth isn't her blood son). If the accident "was no accident" and Callista was the sole survivor with as powerful, careful, and paranoid of a planner as Alexander, he arranged it to ensure that Callista survived, and that she was the only one that could. He banged both Elena and Yvette, who had given birth before Callista even fled (pregnant with Kane upon flight, and age gap between Olivia and Kane).
I'm not sure where you're getting that Alexander ignored Alessandra, as she also went through the trials, but she apparently sees her older sisters as being too soft, as they complain about having "invisible scars" while Alessandra views it as "Oh just boring past events that were of no issue" (in so many words). We also learn right off the bat that she's a mathematical genius, and thus runs all of KG's accounting, which apparently isn't a managerial position, which she states she prefers that lack of drudgery-- I mean: responsibility.
I'm also not sure how you can conclude that Dominique and Veronica, who have self-described as being "cruel" and even performing/perpetuating evil, would therefore not refer to herself as "cruel goddess". Furthermore, Callista is supporting Grymm, but Elena will confess that an evil Olivia will effectively make "Callista roll over in her grave after seeing you take this path," so who would a good "good Callista" support in the Gambit?