Let me take a stab here.
Medically I would love to know why these synkenesis symptoms occur. As you noted, a large smile would close the eyes. I believe I understand the nerve signaling but not the cause. For example, the facial nerve has five branches for the eye, forehead, neck, mouth, and cheek (I think). The facial nucleus in the brain stem is responsible for coordinating which fibers fire. It's basically a single nerve (facial nerve) with thousands of nerve fibers which are controlled by the nucleus via the five branches. Having greater control of these fibers is why some people can raise eyebrows, nose flairs, etc. This is not identical for all people, hence some people may naturally close their eyes when they smile. I can easily understand how the eyes will close, during a smile or for that matter how the neck muscles will tighten. I would think this is because the facial nucleus is firing the nerves and when you are forcing the motion the brain is more confused so it send more nerve signals. However, why is the brain stem firing these extra nerve fibers? Trying hard, makes some sense. Think about a normal person that tries to force a huge smile. Normally this person's eyes would start to close. In the case of Charles, as he tries to force a larger and larger smile, eventually the neck muscles tighten. This would makes as much sense as the eye, since the eye, smile and neck are controlled by the facial nerve. I have NOT seen a medical journal on this topic. They discuss how Botox could be used to dull the firing of these extra fibers (same procedure to get rid of wrinkles) but it's unclear why the brain stem is firing them in the first place.
In my case, my masseter nerve was grafted to my facial nerve and the brow and neck branches were disconnected. Hence, tightening my jaw will create a smile and start to close the eye. As expected. However,. The group that has a weekend facial nerve, the firing of the nerve fibers is either by the brain stem, or firing on their own.
Interesting. Thoughts?