Those types of cracks don't occur because of the rider’s weight, they occur because the spokes were over tightened. That's why your quick survey came up with cracked rims regardless of rider weight. A percentage of their wheels are coming from the factory over tightened. When the OP's wheel was new the spokes were not tensioned properly (too loose or uneven tensioning) from the factory resulting in the wheel going out of true repeatedly. If the mechanic would have retensioned the wheel properly back then the problem would have been solved.
What this whole thing suggests to me is that Mavic has a bit of a QA problem on their machine-built wheels (big surprise). When the mechanic rebuilt the wheel he over tightened the spokes, simple as that. Bigfred, we may have to agree to disagree on this one.