You're not likely to find a computer science major sitting in a class where they tell you: "Now, this is how you clear the cache in IE. Here is how you fix a Windows 7 bootup problem. This is the software package you want to buy to do your taxes. Here is how iTunes works. Etc." The high school kid working for the Geek Squad is likely to know more about that kind of stuff.
Kind of like an excellent mechanic may be an awful race car driver.
Ok, say CPD didn't do it. Whoever did, we agree that they're smart enough to get in there without detection. They're smart enough to plant the stuff so that it requires sophisticated extraction techniques to access.
And this person committing such a crime, with all this tech savvy, isn't smart enough to make sure the timestamps add up? Wouldn't that be the FIRST thing you'd be concerned about, in order to make him look guilty and in order to cover your own tracks?
"I learned long ago, never to wrestle with a pig. You get dirty, and besides, the pig likes it." - George Bernard Shaw
Also, the FBI was looking for things on the PC that might indicate things like pre-meditation, things leading to motive, things being wiped, etc. JW is a security expert, he specializes in looking for things that have been done to computers in addition to things that are simply on computers. I think they were given different tasks and as such the analysis of the data they looked at in relation to those tasks led to different conclusions.
Hoisted with his own Petard