Hello,
Some version of this question has been asked over the years. Just to make sure I have arrived at the right solution...
It seems the best thing to do would be to make a separate reservation for the Domodossola(I) to Varenna leg using, say, Trenitalia. If I am careful enough I could try to be on a train that goes via Domodossola so I don't have to switch trains. Although I might need to switch seats in the train since I will have reserved seats Domodossola onwards which might not be the seats I am sitting on to begin with.
I looked at the option of using SBB to buy a comprehensive ticket from Wengen to Varenna. Some posts on some forums suggested that as an option. What I find is, while the Italian schedule/trains are loaded on SBB, there are other fare or booking related issues that complicate the whole business.
Any commentary on my assessment will be highly appreciated,
Thanks!