I wonder how old can Courage and Monkey be in that flashback panel. Pretty old for their respective species, i'm sure.
As for the traitor thing, i'm pretty sure Point out Mandy would be too easy, additionally (someone said it) she'd have no interest in doing so unless heroes would become her ennemies, but that would be a switch side and end up with Mandy crushed (even if not killed), not the betrayal from within that could have taken down Megaville. That's true, from the looks of the FBI agent in the current PPG storyline, they blindly trust their heroes, so an infiltrated agent would be devastating indeed.
Someone wondered about Pain being an infiltrated agent within the Grim House, explaining why the map was stolen and ended up in possession of HIM. That's an interesting possibility actually, which would explain why Pain is so zealous in serving Mandy. Like that person said, the "traitor within" line end up precisely when the panel zoom in on Pain serving Mandy some wine...
Mr Guy Miah : I think you saw Mr Bison's outfit in Courage, but i'm not sure. Several elements tend to be part of a sailer or something related to boats. Like his "smoke thing" (missing the english word here xD).
I also wonder, like others, how did Buttercup lose her eye. But i also notice her new outfit is a martial arts themed one. Undoubtedly she followed Seinsei Jack as his apprentice or more. And since "those who live by the sword die by the sword", she most likely got hit hard during one of these battles. That'd be interesting, since it would be the most powerful way to force Buttercup to calm down and thing before rushing.
I don't think Olga will be any problem in the future. After all, she did try some test on Dexter at first, thinking he was responsible for her brother's demise and wanting revenge in her dark thoughts. But they were childish at best, and when the raid started she was completly surprised and helpless. And that's precisely when she witnessed how all of them, including Dexter, would do so much effort in protecting peaple. If they survive the current storyline, i'm sure Olga is going to confess and ask for forgiveness. So she can't be the future traitor.
Peaple have speculated the possibility of a mind controlled traitor. That would be interesting too, but the possibilities would become endless. Still, i think that a good mind-controlled traitor would be XJ9 : Bell's father has much greater scientific expertise than anyone on Earth, it would be so easy for him to hide a virus within XJ9's system to stay there even after she's freed from being used as a portal generator.