I'm referring to AVAILABILITY of the toilets, not FUNCTIONALITY. Even a defective toilet is "available" as a simple holding tank even in cold weather. The composting and evaporation may not function effectively in late fall or early spring, but there should be no technical reason to close toilets except if snow is blocking the door. Once that happens, or if the Rangers decide to close them for the season, then the area is just like any other winter place in the Sierra without toilets. All they have to do is let people know the toilets are closed so they don't count on them. They could also leave a box of wag bags near the toilets for emergency use or for people that prefer them over a toilet. Keep everybody happy.
I never promised to provide a new toilet design for Mt Whitney. The Long's Peak report linked to above has engineered plans and actual costs for you. Hopefully that will convince you there is at least one such design that actually does work. Phoenix toilets has other designs that are all over the national park system. The main adjustment for high elevation is building a well-insulated holding tank compartment and using the best materials. This is not rocket science.
You can point out the challenges they have overcome at Long's Peak mentioned in the assessment report, but the fact of the matter is the system is a success, the Park Engineer is happy, the hikers are happy, and the Rangers do not have to do the maintenance work so they are assumed to be happy as well.
