We don't really need to get the warden in there though, do we? All we need to do is get our current body in there. We can just puppet it there and use the Warden to give it the authority to be there. And with the Warden's knowledge we'll also know when the Anima lens is operated, so we know when we can use our puppets So our biggest issue would be to find a way to kill the warden quickly and without anybody noticing us.
Do we know where the Warden (or perhaps the highest ranking officer of the night shift) sleep? Do we know if he has any windows or any other ways to enter his room (chimney, vents, etc.)? And do we have any leftover poison we can use for a second poisoning? Or something else that could be used by a relatively small creature (e.g. raccoon) to quietly kill someone? Like a scalpel or something poisonous? Do we know if the warden have any habits that would render him susceptible to poisoning, e.g. drinking a glass of alcohol before bed?
I'm thinking, we pretend to go to sleep, we telerot to the bird, we scout the rooms to find our target, we telerot to the snake, we slither out to a window, we telerot to the bird, have the bird pickup the snake and bring it back to the forest to supply it with the rest of the poison, then we fly it back to the target's room. Hmm. Then again it's winter so a window being open is not as likely. Maybe a chimney? I bet officers have fireplaces in their rooms.
Also, now that I think about we don't have to leave the body in a pristine condition. A stab wound through the heart could be bandaged and concealed beneath clothing. We just have to be careful to leave the face unharmed (or no more harmed than something that can be justified as "I cut myself shaving"). Maybe that raccoon could come in handy, what with the ability to wield tools. Maybe we can fly it up to a window, have it quietly open the window and then get in and... I dunno, inject the Warden with some dangerous detergent or something. Or cut an important artery, supposedly one falls unconscious almost immediately and dies within a minute or so of these arteries being severed and we can target one that can be covered by clothes (e.g. the legs). Or just stab him through the heart.