Is your human character immune to Sion's charisma? I'd say partially. Zel is human mentally, but generally he's been stabbed in the back enough that he mistrusts people. So Sion's charisma might move him up to Zel assuming he's a generally good and safe guy, but not breaking from his 'I band to people when required only'.
Any blackmail information that Sion could find via digging Most of Zel's screw-ups were open enough that they wouldn't work well for blackmail. But if I think of anything, I'll let you know. He is a mercenary at heart, and mostly the fact he's been stuck in a single city for years has kept him tame.
General Opt-Out for Asruld Shenanigans: No
Hearing Asruld?:Sure. Zel's dealt with scary things possessing humans. (Generally it has not been good experiences.)
Sword: Sure! Zel is perfectly vulnerable to magical bullshit, even if non-magical physical swords don't work too well on him.