The baby was Cassie's (with Hart) - Cassie having his child while Dinah could not was one of the things which sent her further into a point of no return, along with the period (if you can find any of this, please watch some just for how ludicrous it is) where she was left with scars from setting a fire to keep Hart with her. This went on for months, with Wendy Moniz forced to hold a napkin to her face rather than the makeup people having to spend a lot of time and money.
Dinah was insufferable for Moniz's whole run, but the main difference for me between the first few years and the last few is that in those last few she was meant to be damaged, bitter, vengeful, and I could understand that and feel for her. In the first few she was supposed to be a heroine, and it just did not work, ever.
Grillo was cast after the actor before him left abruptly. I think if Hart had ever had any value to the show by those years beyond a piece of ass, they would have tried harder to find a better actor, but he didn't, especially after Michael Zaslow was fired.
I did like Morgan Englund but you aren't wrong. There's some anecdote, I have no idea if it's true, that someone met his mother at a party and she told them she never thought he was going to be an actor either.
San Cristobel wasn't around that long considering how much it generated controversy at the time for hogging airtime and never going away - I think summer 1999 - fall 2001 (probably wrong). If there were any black people they were probably just extras. There's one somewhat infamous scene where Cassie gives her tiara "to the people" - knowing P&G by that point, if they had any black actors they would have put them in that type of scene.