Stresa to Domodossola will be a regional train and it might not show up on the schedule until close to June 9, when all the regional schedules get revised. The rail line south of Stresa will be closed due to a construction/repair project. So I would wait 3 weeks to see if any trains show up on the schedule. Worst case, there should be buses from Stresa to Domodossola.
From Domodossola, Swiss trains will get you to Wengen. I see trains at 9:58, 11:58, 13:58, as well at times earlier and later. You can take the 3 trains mentioned all the way to Spiez over the scenic Loetschberger route.