our imaginations try to fill the gaps and connect the dots. This can lead to scenarios and abstracts that are unrealistic, but feel true and can be horrifying

I wonder what it would be like in a situation like this

Would he be able to like slowly walk himself back to safety while he keeps dying?

I agree i found the Jack saying that "the creature eats its way into the brain" being interesting and a piece of lore about the midnight demon that i hadn't heard before.

Nope

The doctor even has a feeling it could still be there.

It's a creature with no identity, no form, no actual appearance other than a shadow. No voice of its own, no confirmed species but definite confirmed will and malicious intent. The doctor couldn't beat it, no one could. Only a sacrifice to take the possessed victim away while the doctor, incapacitated, could only watch as the people around him became overwhelmed with paranoia. To feel so weak and unable to do anything, the loss of control. That, is what true horror is