The safes in a room can always be solved with everything within the room. There is one that you need outside knowledge to open, but there is rules all those safes follow that allow you figure things out slightly better. You'll slowly learn the hints to help you figure out those safes.

For the boudoir specifically I'll say do not think hard about it. What's in the room, how does that become a 4 digit code?

The schoolhouse rooms are really great at providing you a huge amount of clues to many of the puzzles of the game. Some of them present alot of information that can be difficult to parse, don't try to memorize it all at this moment but be aware it. Some of the final classrooms are very dense so if you want to spend time learning it all you may need an hour to try and absorb it.

The passageway is a tricky room, it's useful, but very expensive at 2 gems, and never provides anything other than power routing. I'd say use it sparingly unless you're very flush with gems. The resources become more manageable as you learn more of the manor and solve some puzzles.

The number 8? Might be important. This game is kind of about trying to remember lots of things and trying to smash them all to complete the proverbial picture. Alternatively keep a notebook and try to read through it and see if that helps you piece things together.