I'm of the opinion that few, if anyone short of Celestial power levels could break 1 ton adamantium handcuffs in less then 5 seconds, let alone behind their backs.
If given time, maybe the big hitters like Superman, Hulk, SS, DS, Thanos, Thor, ect. could break them, but I seriously doubt it.
As for Thanos being bullet proof, I know its not canon from the comics, but I have the Marvel Universe Role Playing stats for him right in front of me and he has Monstrous level resistance to physical damage, which is more than enough to resist bullets. Whether its been shown in comics or not, the writers of the game must have been given/shown/lead to believe that he is most definitely bulletproof.
Oh, and its been a loooong time since I read the comic, so I could very well be wrong, but I seem to recall Thor breaking his hammer on a Celestial while he was attempting to break into it.