Laurel - Männlichen doesn’t open until May 25. And the Grutschalp to Murren train will be closed 6-31 May this year for putting in the new train cars, so to hike Murren to Grutschalp you have to be willing to do it roundtrip with no train return. That’s fine to do, just a note to double the distance.
That being said, I think Gimmelwald would be fine for a few days in May as you’d still find things to do in the area (Lauterbrunnen, Murren to Gimmelwald, parts of the North Face hike, the Lakes, Schilthorn, etc) but I probably wouldn’t stay in Murren since the train is going to be closed. Do Gimmelwald if you don’t mind the bus connection or do Wengen if you want more access to restaurants & shops.
For 9 days, maybe do something like 3 nights Montreux (Chateau de Chillon/vineyard hike from Chexbres to St Saphorin/Boat Cruise to Yvoire/etc), 3 nights Gimmelwald or Wengen (things I mentioned above), 3 Nights Luzern (Rigi/Titlis/Stoos/Lake Cruise, etc), Return to the airport from Luzern on departure day