I'd also love some clarification on the "ghosts need to acknowledge you to affect them" thing. Does it make them immune to everything, or just to you affecting their manifested forms? Again, mostly curious about how that affects possessed people.
If they're manifesting a form out of ectoplasm (i.e., they're "insane ghosts", often called Specters) in order to interact with the real world, then they can be affected physically--even with bullets and knives.
If they're
not manifesting a form, that's when they're in "ghost mode." Invisible or mostly invisible and hard to even detect without Ghost Speaker or being an ectomancer. This is when it's nearly impossible to affect them in any way. You have to get them to "acknowledge" you like in the beginning of Ghost Story when Harry had to call Agatha Hagglethorn's Name before he could smack her with his ghost dust...IIRC he specifically said even if he'd dumped it all over her, if she hadn't acknowledged him first it wouldn't have much effect.Once she'd taken notice of him and (from her perspective) he became "real" to her, he could then work magicks against her or use his ghost dust.
Before he used her Name, Agatha was smothering babies and thus interacting with the physical world, but she could get away with this without manifesting a physical form because that kind of action had to do with why she was dead. In a way, the babies had already been "acknowledged" by Agatha because of the reason behind her being a ghost in the first place.