i agree with you, i have a feel negral is cliff's son.
feeling*
I think, based on Nergal and Mephisto's conversations, Cliff wouldn't survive coming to their dimension. Just like how Nergal and the others can't settle in others.
How Cliff managed to send Leon without harming him beats me. Leon is probably an exception., like how he arrived in Cliff's world out of nowhere.
I think Cliff didn't send Leon. I believe both Cliff and Leon got transferred to the current dimension. That's why Cliff is trying to find (?) A way home when he found Leon. And yes, Leon could probably be an exception. That's why he's still alive.
So based on that theory, it's either Cliff was able to return home... Or he's gone gone now （/TДT)/
I still feel like Negral is somewhat related to Cliff.. like.. WTF HAPPENED TO CLIFF IF THEY CAN'T STAY IN OTHER DIMENSIONS?? IS HE NOT FROM THAT DIMENSION? dude I'm getting more questions than answer right now