I haven’t seen the Haute Route program but can share the following thoughts pertaining to your question:
-For the strength you want to work on, if the Haute Route plan doesn’t already have gym specific workouts then I would add in those exercises to the gym workouts in the program
-For technique, since going downhill will work out the muscles you’re training, I would eliminate the most “ski specific” exercises but still do the remaining exercises in the workout plan for that day
Saturday gym workout contains plyometric squats + core. If you’re going skiing on Saturday, still do the workout but remove the squats and just do core (and whatever’s left).
I would think of it like “substituting” skiing for the leg specific workouts
Granted, if you have a huge ski day and your recovery/energy to do the remaining gym workout is dead, then adjust accordingly.