While granting that defective spokes do exist, and can cause problems, I think your problem is spoke tension. All the spokes need to be replaced, and the wheel tensioned properly.
The rear wheel definitely needs to be rebuilt, no matter the cause. But tension doesn't explain the mode of failure. Breakage at the head is virtually unheard of, and wouldn't be caused by improper tension. Properly cold headed the head, is as strong as the rest of the spoke, so 5 identical failures there is significant.