Are all these areas accessible by train?
Yes.
Swiss Rail Pass or half price?
Depends. You need 10 days of travel. the Swiss Travel Pass comes in 8-day variety. Last year, in the spring they had a promotion of buy a 10-day pass for the price of the 8-day pass. Hard to say if they will run it again this year. How old are your adult children? If they are under 25, they can get a youth discount of about 30%. Otherwise, you have to weigh the advantage of perhaps lower cost with the Half Fare Card vs the convenience of just hopping on the trains without having to buy tickets. It is getting to buy tickets with your smartphone these days.
Is this realistic?
Seeing the Matterhorn with a one day window is crap shoot, depending on the weather. If it is clear, it is a 1 1/2 hour train ride from Brig to Zermatt each way. I just got back from Zermatt and the Matterhorn was in my face everyday with a high pressure system overhead and clear weather.
Going from Brig to Spiez, take the BLS line over the mountains rather than the fast way. Stunningly scenic. See my review elsewhere on this site. https://community.ricksteves.com/travel-forum/switzerland/ride-the-bls