From all of the “missing” cases, which ones do you think are still alive today?

The only one that comes to my mind is Selena Eden

Maybe Sabrina Aisenberg. I'm fairly certain that Marlene Santana is still alive. I think it's a 50/50 chance that Jim Wood is still alive. Same with Gordon Page.

I concur on Marlene Santana and I think she might be the only one I could confidently wager is still alive.

Jacqueline Castaneda. She would be 18 years old today, which is around the age when an abducted infant might become suspicious if their "parents" cannot provide any documentation like a birth certificate to prove she's their child.