The guy escorting Robin in page 1 isn't Sabo..

This guy is:

I see no reason for Oda to draw him grown up if he was dead.
As for reasons/possibilities why he didn't show up to save Ace....there are millions. Here is 1:

Maybe he had faith in the WB pirates?

edit: there is no crossed "S" on Ace's arm. We all know Oda..he's telling us that Sabo is alive