Just adding more credence to the idea that this body is Steven's--
In none of the photos I've seen is Steven ever wearing a watch. However, I discovered
this photo collection online, presumably set up by someone in the Koecher family. I just stumbled upon it randomly while looking for info on Steven and his case. There are lots of photos of Steven and his family. His mom and dad are almost always wearing a watch, and two of his brothers also frequently appear to wear a watch. Because of this, it's likely that Steven could have been wearing a watch on that day.
The other thing that gets me is the description of the "underpants". Usually you would expect a brand or further description. As a good ol' Mormon, Steven would have been wearing his garments, which do not have a brand or anything else to identify them, and would qualify as "underpants". That would be a good reason for why so little description is given.
Edit: As for the multiple pairs of pants, I think it's more suspicious that they would have multiple pairs of pants at all. Especially ones as seemingly mismatched as dress pants and jeans. He definitely appears to be wearing dress pants on the video, though. Perhaps the jeans were worn underneath to keep him warm? Alternatively, someone could have put them on him afterwards?
Edit 2: To add more possibility to the two pairs of pants, there's a large possibility that after Steven left at 10:30 PM the previous night, he didn't go back home. This means he slept in his car, and definitely wasn't keeping it on--thus perhaps the multiple pants were to keep him warm in his car? Furthermore, we know he liked to bulk up, considering that multiple coats were found in his car.