He has returned.
It's unlikely because then Ash would have to battle someone else, and there's nothing pointing toward him battling Steven or Diantha at all.
Originally it seemed like he would have battled Lance, but now it seems like Iris vs. Lance is more likely. There's really nobody else for Ash to battle prior to Cynthia/Leon.
and you don't think the writers aren't known for throwing curveballs?
they've done it in the past, several times.
that's the thing, you can be lead to believe that Lance/Alain and Cynthia are the ones Ash will face to get to Leon because of the hype, but what if it's a trap and they have him fight Steven and Diantha (regardless of order) instead?