Yesterday, an anonymous commenter said that it's possible to skin the séance in Nancy Drew: Message in a Haunted Mansion. It turns out that is correct! What you need to do is talk to everyone, then solve the Diego puzzle on the stairs. Enter and exit Nancy's room to trigger the fire in the sitting room. Once you put out the fire, Abby forgets about putting on the séance.
The fire scene takes about 40 seconds, and the séance takes about 90 seconds, so that is a timesaver! Thank you, anonymous commenter! Also, curse you, because I had my speedrun all finished, and now I have completely redo it to take your tip into account! Grumble, grumble.
But wait, there's more! Amy commented with an itinerary for getting through the game quickly. Hmmm...that's basically what I'm going to do, except I start with talking to everyone. All the stuff with the tiles and the attic will be a "timewaster" of sorts. That way, I don't need to set the alarm a second time.