I like the theories that involve Gaius Julius, but that's a personal opinion, not a linguistic judgement.
On a related note, I heard the name 'Caesar' in ancient Rome was pronounced more like the German 'Kaiser.' My source was Colleen McCullough, which I know is silly, but I tend to believe it. I also heard that the emperors following Gaius Julius took on 'Caesar' as a title out of respect for the man (that seems weird to me, was there a precedent for that?). I assume that lead to the German word 'Kaiser' and the Dutch counterpart. All that sound about right?