If only one file had a bad time stamp, bad watermark, and missing cookie file then I'd say "yep, that file was tampered with or planted". But since every single Google cache file has the same issues, I say "there was a systemic problem with Google or the laptop, but those files are valid.". The theory I'm going with: BC was using a private browsing feature of his browser that malfunctioned and, using today's expert witness term, "dropped" a bunch of files in the cache directory.
I'm not saying that this is a fact, but it has occurred to me that some drives hold write data in a memory cache and flush it to the drive at specific intervals, when the cache becomes full or upon proper shutdown. For this reason, you can corrupt a drive by just powering the computer down without doing a proper shutdown. Some of the data that the system was holding in memory does not get written back to the disk, and you could cause problems with the system.
Is this why the files were munged? Was this an interrupted cache flush?