squash,

"I know any hit can cause a concussion, but the pure statistical probability of a concussion probably increases as the amount of force increases."

This is not true. It takes a minimum force to cause a concussion. Research suggest 50 to 60 G's as a concussion minimum. Many can tolerate 100 or more G's without suffering a concussion. The lesser impacts would be considered sub-concussive impacts. Alone, they are not a problem. If you engaged in an activity where you were subjecting your head to 10 of these a day over an extended period, there may be a concern.

But, the impacts you suggest would not even rate as sub-concussive impacts.

An impact can cause pain at the site due to trauma to the nerves in the skin without ever causing any trauma to the brain.

I think you have two issues to address.

1. Your neck. Stretches do not resolve cervical vertebra injuries. It takes careful and subtle manipulation and mobilization of the vertebra to help settle them into proper position. Then, it takes discipline with sleep posture to maintain that. Straight head and neck positions during sleep are important. If you feel stiff in your neck when you wake, you need to resolve that.

2. Your anxiety levels. It sounds like you look for symptoms. But, as you engage in your days activities, you stop looking for symptoms and they go away. You may be training your brain to focus on symptoms that are meaningless. You are training your brain to become anxious when there is any contact with your head. Stop that and get on with your life.

Hits happen. Move on. Just choose the level of risk you take. Contact sports, and physical activities that pose a direct risk to a head impact can be avoided. Living life cannot.