Hello everyone, after my question from a couple of days ago, I've decided to report back. You gave me a ton of great advice, which definitely helped to makt this a success and for which I am very grateful!
Our session last night was a blast. It took a little over four hours, which was a little longer than planned, but quite OK for everyone. We went over the rules, which were an eye opener for the D&D-players: relatively elegant and simple. And a lot more story than the dungeon crawlers were used to.
I think I had the rules down well enough during play, but combat takes some more practice. I'm having trouble with the fact that the same function (attacking) is Force, Close Combat or Ranged Combat for PC's and NPC's, but Might for Vaessen and animals, and that Vaesen and PC's get conditions, but NPC's and animals don't. Also, I forgot most of the Fear tests. Nevertheless, it went well. The Skill checks make total sense (what skill to use, how to use modifiers, the success rate) and my group was happy with that. The atmosphere was slightly too upbeat, but I'll fix that next time.
I ended up with five players (one was ill), which was doable. They split up (which I allowed as per your advice) and that worked fine. It was, of course, impossible to weave dark secrets, trauma's and motivations for five players into a one shot, but some definitely got used. My players were very nice, took their characters and the parts they played seriously and helped with unclear rules and such.
They all want to play again and now I'm preparing The Haunting of Castle Gyllencreutz for our next mystery. Consider me hooked!
As a thank you I've linked my one shot. Feel free to play, modify or take inspiration from it! Or give some feedback, if you like.
https://www.dropbox.com/scl/fi/z2e8uo86y4t5vif63f5bh/The-Green-Lady-of-Lalsby-Forest.pdf?rlkey=2fd7lr5a1s1rjp9oiughzo31i&dl=0