Does this actually hold up as a horror film?

I'd say so yeah.

It's a who done it mystery first and foremost but they do make efforts to make a spooky ghost story.

no.
it's a lazy film and the camera work makes you nauseous.
the word adaptation is very loose, the writers just used character names and a small portion of the murder plot.

the scariest part of this garbage is Tina Fey's "acting"

Not really. It has a creepy atmosphere and jump scares but that's it. More of a mystery thriller than horror.