The cam follower broke? Do you think the shim/collar came off breaking the follower and the subsequently the belt?
Actually, just the opposite…
The belt became loose, the valve was down and cam lobe was down. When the piston came back up, it pushed on the whole assembly, cracked the rocker arm, the opener shim fell off and so did the closer. They will simply rattle off without the rocker arm in place, hence the reason why clearances are so important.
So the big fifty thousand dollar question is… how did the belt come loose? Maybe it was simply too worn? Most people replace belts at 15k miles, so 28k miles is a lot. Maybe my theory of the mobile roller coming loose on its own or the rollers not in line with each other, causing edge wear?