Marks On 1964 Kennedy

Here are some better enlarged photos.

Might the marks on the neck be a clashed die from the lines on the shield?

These marks on your half are not mint striking errors, sorry. Both obverse and reverse, those marks are all post mint damage ( PMD). They can be from clashing with other coins, or some other contact with another hard surface, anything is possible and we'll never know the exact reason. There is nothing in the minting process that would account for these marks.

99% of the time incuse marks are just coin damage. 1% of the time they are a mint error. Stuff happens to coins. I agree with the observations, that these are all damage to the coin. The heavier the coin, the more it can damage another coin. We see more bag marks on cents. But more reed transfers on the larger coins. So I feel most of these incuse marks are caused after they are struck. More of a handling issue right after the strike at the mint of them falling on each other. So they probably happen more often there. Just my observation.