If you reserve seats on the Golden Pass trains, you can't then switch in mid-stream. (Although, I guess you could reserve on two differernt trains).
However, if you don't reserve seats, there is nothing to prevent on and of travel on the day that you travel.
There is some kind of track work on that route near Montbovon in March or April.
Don't remember the details, but it ought to show up in the SBB timetable. A bus is substituted for part of the trip.
<<"Also, heading from Montreux to Wengen May 2 and would prefer to see other sights and not backtrack on the train. Any thoughts on the best route to Wengen that wouldn't duplicate our route from Luzern to Montreux?">>
A combination of this map and the SBB timetable can help. I think you may have used the SBB timetable already, from you comment about time line.
If not, this will help. Use "Advanced" to put in more than one "via."
Make use of the via box, after you look at the map. On this map, if you zoom in to zoom level 32, the train tracks appear.
You can see routes through Lausanne, Fribourg, and Bern, back country routes via Montbovon+Gruyeres, and a route through Martigny and Visp ( faster) or Martigny and Brig+Kandersteg ( more scenic.).
Along the northern routes, Gruyers and Fribourg could make an interesting break if you want to take the time, as could Bern.