Any plan that involves a lot of body swapping, particularly at long range, will probably use up a lot of mana. And I'm not sure if we generate it on our own, do we? Is Esme willing to keep us charged up?
We can hop back to our human body and check our ring as we walk our human body back to the library
You carefully park your hawk body and then leap back into your normal vessel, awakening where you left yourself. You check your ring. Still mostly full, so it seems like mana is only drained when you leap, not to maintain the connection. Useful.
You return to the library, heading in through the back, and find Esme arranging your hawk body in the prepared specimen area along with the three skeletons of the soldiers. You tell her that these jumps take mana and ask if she's be willing or able to keep you topped up if required. She says she will, if its needed.
"I've been looking at the various poisons available to us for this whole prison thing. I think we might actually be better off with an anesthetic. Most poisons are not exactly subtle, but with a heavy overdose of anesthetic they have a much better chance of simply going unconscious and dying, as opposed to vomiting and convulsing and dying over minutes or hours. If you can get one while they're sleeping, it might be effectively seamless to kill and animate them; like they went to bed one night and woke up the next day as usual. Then you get your talking done and dispose of the body. Maybe make it look like they fell off the roof or something. Just nothing immediately obvious otherwise they'll probably wonder why the body is already cold immediately after death."