It makes total sense that if you're not into the whole Witchcraft/Medieval thing, you probably won't be that into The Church. Although on the Soavi tip, I'm surprised that you haven't seen Dellamorte/Dellamore (unless you have and forgot it was made by him), which I think is his best film by far.
I tend to think the problems you mention with Opera apply to most of Argento's films. They invariably have one or two really outstanding set pieces but it's the dead spots between them that ultimately let him down for me. Without wanting to sound like I'm belittling him (and in some ways quite the opposite), I think he'd be great directing quite high-end commercials or doing stuff for a gallery. Anything that lets him focus on his obvious visual flair while not having to bother with narrative or plotting, his obvious weakspot. Without currently being able to check, I'm sure he's done some promo or backdrop work for Agnes B.
My internet connection is really slow right now, so I'll have to look at that Bava doc later. It doesn't look like one I've seen before. Thanks!
Oh yeah, and it turns out Sabrina Salerno was in a film called Delirium, not Stagefright. I've no idea how I got them mixed up. As far as I can gather, there isn't any connection between her and Soavi at all. Maybe my wishing she was in more films than she is, extends to me imagining she's in movies that she never had anything to do with. Pure wishful thinking.