I don't think safety is the primary concern, especially now. They are in the window for a search, as
@Trackergd noted recently. I think cadaver dogs and perhaps other means of detecting remains (and perhaps a cooler) operate more effectively in warmer weather.
The investigators represented to the court that SM's remains were in a remote and mountainous area near the Puma Path residence, and they would not do that lightly. They have a good faith reason to believe they are close to finding SM, and they will be following up with a search when conditions are optimal for success (if the search is not already underway).
I agree though, that although BM was fit, strong, and acclimated to altitude from his firefighter training, he could not have carried SM's remains far uphill. I speculate that he had prepared a spot off a rough track - perhaps the remains of previous firefighting activity - then took the remains up the mountain via the ATV to a point on the track ABOVE the disposal site. He could then carry the remains more easily, downhill. All MOO.