Is it at all possible the Sabo is Trafalgar Law? Maybe this flashback is about why Trafalgar Law is helping Luffy and not really about Ace? If Sabo done something to piss of a Tenryuubito it would explain why he left and why he has a different name now, the Revolutionaries/Garp/Dadan could of somehow got him to North Blue and given him a different name to protect him from the Tenryuubito/Marines (although why the Revolutionaries/Garp/Dadan would help him like this I have no idea). The only REAL problem with this idea is why didn't Luffy recognise him (although this is Luffy we're talking about, they only knew each other as kids and both times they met they were in the middle of a fight against the marines)

Then again maybe the above is all a waste of text and it's just me futilely trying to work out Law since there is obviously more to him (or he knows more) than he is letting on.

As for Dadan my prediction is that she is somehow connected to the Revolutionaries and thus Dragon, which is why Garp is leaving Luffy and Ace with her. Since I don't think Garp would just leave his grandson with a group of random bandits.