It's not really strange theories as much as it's people sharing a lot of gossip, rumors and scary stories. Sadly, Stacy's case is textbook typical of how people get lost in the wilderness.
These are strange theories.
Anyone who tries to figure out what could happen but can rely on facts (however little of them is available) will end up with a theory.
Anyone who tries to do same thing but is unable to figure out what is true, what is hypothetical and what is just straight up gossip and misinformation will end up with a strange theory because he will try to solve a unexisting mystery.
As far as these articles go it looks like NPS and volunteers did all what they could do. So what is there to hide?
Here's a link to Google Earth, to the location of the Sunrise Lakes at Yosemite. View it in 3D. Look for the green marker that says Sunrise High Sierra Camp. It's on the other side of a nearly bald mountain ridge.
Google Earth
Here's a screen cap in the attachment below. In 3D you can see the camp sits on the other side of the ridge from the lakes, down lower. There's no way the lakes can be seen from the camp area. The information that the people at the camp could see her at the lake was probably just a wrong version of the story that has been repeated many times.
The distance from the camp to the closest of the 3 Sunrise Lakes is 1 mile. That's quite a distance and according to people who have hiked that area, it was probably getting dark during her trip back to the camp. She didn't have a flashlight, she probably wandered off the trail without realizing it and by the time the sun came up, she was lost.
So she came back to the camp from riding through the park with her group, rest for a bit and decided to go back and take some pics of the lake which she likely saw before, while on horse.
She was wearing her flipflops and dad told her to change it into proper hiking boots.
She went there alone and people from the camp saw her while she went through first part of that trail... likely till before she disappeared behind trees after turning right.
Sometime during that walk she met 77 yo man and they walked together for 20-30 minutes which I guess is something between one half and third of the distance between camp and lake.
We don't know if that man was horseback riding with her group before, if they knew each other or if Stacy was aware that he went towards the lake before her, he just decided to take his way back at some point - either because he was tired or cause he wanted to go back before the dawn.
Some time later other group went through the trail where they logically should met Stacy at some point but the one man asked about it reportedly never saw her.
Then people realized that Stacy disappeared and searches for her went fruitless.
This for me brings different questions.
Was there a possibility that she lost something while horseback riding and went to "take some pictures" as an excuse to look for that?
Was she riding horse in flips or did she wore hiking boots?
Was her attempt to walk there in flops caused by her possible naivity or were her feet possibly hurt from wearing those hiking boots and that made her reluctant to wore these again?
How fit was that 77 yo? How fast did he walked? I can't imagine him being fast hiker at this age but if he was well rested before and quite fit he might walk as fast as relatively tired 14 yo... but if he wasn't very fit nor strong and walked slowly then it might be telling if Stacy slowed down to walk with him or if she was already exhausted.
I would expect to find answers for all of these and few more in case report files, at least some notes had to be taken, right?