Yes, honestly, if you're impersonating somebody it would only make sense to take the original hostage. Or else the cover would be blown. She squints because denying that basic principle is exactly what a body snatcher would do.

Ava shakes her head, a bit in denial and a bit trying to shake off the itching paranoia.

She wants to ask him to prove it, that he's the real Skulduggery. She trusts him, would take his word for most things almost blindly, except she's certain this is not him.

Ava launches herself at the skeleton man, with an angry sort of shout, ready to tackle him to the floor. If it's an illusion, maybe she can shake it off with force!