Occam's Razor.
She was seen hiking up the mountain, alone, by several people. It was unusually late in the day for a hike into the mountains. Her backpack was noted as "very heavily loaded," which is atypical for an experienced hiker. She summited an hour later and sent pictures to her husband. We don't know how long she stayed at the summit, with the times reported she would have been descending somewhere in the range of 4:30 to 5:30, maybe later if she stayed a bit or took time to eat. You don't have to be an experienced hiker to know that descending a mountain at that time of day, especially in the winter, is not optimal.
Her ride with mystery hiker was three days before. She made no mention of him after that. Red herring.
This site has many, many cases of "experienced" hikers who never came home because they misjudged their ability and/or conditions. That she chose to summit late in the day, and descend as it was getting dark, makes it likely she is, sadly, in that category.