Honestly, good on your players for figuring it out and good on you for playing a memorable NPC. They are going to or have already headed to the sanatorium. After that have them check in with the sheriff, either just to update him or to seek assistance for the remaining residence of the sanatorium. While they are in town maester grump rushes in looking for help at the farms, there the pcs find the key to the misgivings and get to go 'haha i knew it.' It doesnt seem like they will be missing out on any of the story or xp opportunities, they lose out a little on the who done it mystery, but only because they solved it quick. I think your fine. At this point a red herring feels unnecessary.