Hello,
I know that. Many things in this game I wished they added or changed in theirs. Such as shoulder flash light and banishing system.
1. If you notice it say hidden entrance so it's not visible. Try how you enter the normal door in game near the that mannequin.
2. Yes, those interaction appears. The mirror one is easy to spot in small maps or ghost room has mirrors. They motion camera one is only for objective variation. The only ghost that appears in front of it is Bloody Mariam.
3. Yes, as you know the game is still in WIP. Those pages are scattered randomly regardless they are available in the game or not. Done that so I don't have to do it again once all ghost are added.
I'm planning to add a way to reduce sanity faster so will consider fake hunt thing and sanity reducing interactions.