Scratch what I said yesterday, about Nancy Drew: Message in a Haunted Mansion. It turns out that there are two times in the game where you need to talk to everyone. First, at the start of the game. Second, towards the end of the game, when you must talk to everyone about gum bo fu.
This means it is impossible to beat the game, without seeing the séance. The séance starts when you talk to Abby, after talking to Louis. (Bit of an odd trigger there, but that's how it works.)
When you do the séance, it skips you ahead to 10:00 PM at night. That's handy, and it saves me a trip back to Nancy's room to use the alarm. I'll keep working on the route, but right now, it looks like I have two options.
Séance earlier: Skip ahead to 12 PM, and talk to everyone. Talk to Abby to start the séance, which bumps me to 10 PM. Get Charlie's key and do the attic stuff. Skip ahead to 11 AM and spy on Louis to learn about gum bo fu. Call Emily, talk to everyone about gum bo fu, and call Emily again. Nancy should be able to solve the endgame puzzles in her room.
Séance Later: Skip ahead to 4 PM and talk to everyone. Hope I time things perfectly, so I can get Charlie's key at 5:00. Do the attic stuff and spy on Louis to learn about gum bo fu. Call Emily, then do the séance. Skip ahead and talk to everyone about gum bo fu, and call Emily again. Nancy should be able to solve the endgame puzzles in her room.