I was getting done with the whole "Ohhh, I'm super evil and mysterious" shtick without Oda actually fleshing him out or giving him any meaningful characterization. He felt like an extension of his design, this vague ominous black blob of nothing. But his introduction chapter, and especially what Oda's done with him since his proper debut, has been fantastic. From a combat, aura, characterization, and lore hint perspective, he's been one of the most interesting characters in the story recently for me. He's really a tortured individual underneath all that darkness, and I have a feeling his flashback is going to be insane.
And yeah, you're right, I don't necessarily mean Blackbeard himself so much as the BB Pirates as a whole. I could see them using Saturn's appearance to infiltrate Mary Geoise, with the ultimate goal being for BB to eventually kill Imu and take his place at the top of the world. It's what BB does, exploits the existing power structure (Warlord, Emperor, etc..), get his foot in the door through deception, and then turn on everyone once he's in a position to seize everything for himself.
As for Saturn's death, I'm not sure how widely that information has actually spread. Even if the higher-ups know, there could still be guards and other personnel who haven't been informed or wouldn't question Saturn showing up in person. The disguise wouldn't necessarily fool everyone, but it might not have to. They'd only need it to work long enough to get where they want to go.