So if Sheila didn't read the bible, Jeremy is guilty? I do think he's guilty but would struggle to make that argument with any conviction!

Of course it shows Bamber's certain guilt.
There is no evidence she read the bible.
However there is evidence above that the bible believes murder and suicide is a sin, and is going against the religions faith. So if she read it, she would not commit murder/suicide in the first place. And certainly not read it during the massacre.
There is also no evidence that people read the pages the bible was open at, after murder or prior to suicide.
The only two options are -
Bamber planted it.
The bible fell next to Sheila. She picked it up with bloodied hands, opened it and put it on top of her own blood.