So I have been reading a little about the case of Anneliese Michel

One of the stranger claims made is that the exorcists believed she was possessed by Cain, Judas Iscariot, Nero, and Hitler.

Leaving aside whether Michel's case was possession or mental illness, has the Church or any saint given credence to the belief that damned human souls can possess a living human?

Two difficulties come to mind:

1) human souls are incomplete humans that relate to a particular body since members of the species man are separate from each other on account of signate matter as St. Thomas notes in De Ente. How then could the soul of Judas which is tied to the particular body of Judas yet to be resurrected, gain control over the body of another human?

2) Demons presumably attempt to ruin souls through possession in order to damn them. The damned souls of humans do not seem to want other humans damned. That he may testify unto them, lest they also come into this place of torments. Luke 16:28. We need not attribute this to charity, it may simply be a desire to avoid the accidental pains of other souls in hell, but regardless the rich man does not seek the ruination of souls as the demons do.

Noteworthy is that in the film The Exorcism of Emily Rose the demons refer to themselves as the one who entered into Judas, the one who possessed Nero, etc. Which makes me wonder if the sources reporting that Michel's exorcists claimed she was possessed by Hitler or Nero might be mistranslations.

I believe your objections are strong and valid.

But you have no special insight into why or if these exorcists said these things?

It's possible the demons said those things. I tend not to believe demons.

We're getting a little deep but isn't there some guarantee that when an exorcist demands the name of a demon the demon does in fact give its name.

No clue.

Never mind the fact that a damned human soul is in prison...

St. Thomas believed that damned human souls are sometimes permitted to haunt the world, since they are no more imprisoned than the demons are. We know of Saints who appear to the living, and there is even a small museum devoted to items touched by souls in Purgatory begging for prayers. It is not inconceivable that providence has some use for damned souls haunting the earth.

He may be absolutely wrong in this, but the belief that the damned sometimes appear as apparitions outside of people certainly does not carry the same problems that possession of a distinct human body does.

True that, I suppose.

Wait, he came up w/that w/out seeing "A Christmas Carol?!"

Yeah, he's not a doctor of the Church for nothing.

It's hard to conclude that Marley, et al. are damned souls -- if Marley were damned then he would not care one whit for Scrooge's salvation, in fact he would want Scrooge to be damned alongside him.

Not true. The rich man did not want his brothers to join him in Gehenna

That's the problem with taking parables literally. The damned do not have charity; they are incapable of willing good for themselves or others; so they do not wish for others to be saved, but rather for them to be damned alongside of them. St. Thomas explains that Dives, knowing that some must be saved, would rather that his kinsmen be saved instead of others, but ultimately would wish for ALL be damned, including his kindred:

http://www.newadvent.org/summa/5098.htm#article4

So Marley might wish for Scrooge to be saved instead of someone else (e.g. Tiny Tim!) but he would rather that all are damned, including Scrooge. I suppose we could understand the story as Marley being compelled by Grace to appear to Scrooge, rather than merely being permitted.

It's not entirely clear the story is a parable since Lazarus seems to be the name of a person not an archetype.

He may also not want the people to be in hell because it will increase his pains accidentally, can not remember where I read that reading.

Isn't this all a bit of speculation?

Not all, but especially with respect to Marley, yes. More than a bit.

Yes, as St. Thomas notes, his pains would be increased more if all his kindred were damned than if some of his kindred were saved. But the damned would rather that all were damned along with them, even though they know this would increase their pains in hell.

Interpreting A Christmas Carol along strictly Catholic lines is not an easy task!

What parts are not?