Could it have been a case of spontaneous combustion?
Not unless she also suffered spontaneous blunt force head trauma.
There's her fractured skull to consider (she was clobbered) and the indentation on the back of her neck (and then clobbered again), a couple of locked doors and some hair that wasn't hers pulled out by the roots, and her missing hat and coat, all of which plainly says
this was a murder.
There's no way around those facts - and in my mind, no possible way to see those as not adding up to the fact that somebody assaulted Elfrieda and inflicted those terrible burns on her.
That she swore she did it to herself, all the spiritualist lingo and so on, none of that negates the fact she did not lock those doors herself, or clobber herself.
So to me, the real puzzle here is why she would want to protect the person or people who harmed her, along with who did it, and why.
And I'm looking hard at Hitchcock as the instigator of the crime, if not the hand that harmed her. I'm also looking hard at the people he had influence over, the people who were in regular contact with him, who lied or acted strangely - Mrs. Hitchcock, Rosenhagen, Kloer. There might have been others, I'm sure there's some we'll never know about. But just the oddness of their behaviour sends up a big red flag to me that they knew damned well what happened to Elfrieda, and perhaps that's because one or more of them were there.
Yes, Elfrieda wasn't in her right mind in the hospital. But that doesn't mean we can't pull some meaning from what she said. Among her final words: "Did they do this? If they did it - why did they do it?" Why, indeed.
And let's not forget that Hitchcock's business at the time was teaching people how to be manipulative. And that his 'recommended reading' list contained a bunch of cultish new age hooey. And that his prior career was acting -and- directing.