I'm assuming a neck knife worn under clothing, even a coat, would be illegal in Michigan.

Can anyone point me in the right direction for clarifying one or the other?

A neck knife in Michigan is a concealed weapon. Even if you have a concealed pistol license, my reading of the law would not permit you to carry a neck knife with the primary intent of using it as a weapon. Michigan’s catchall provision about carrying a concealed weapon, the “any other dangerous weapon” provision is all about intent. In other words, if you carry a knife in your pocket for general cutting tasks, it is not considered a weapon. (unless it falls within the specifically prohibited criteria, such as a double edged dagger)  But if you carry it in your pocket for self-defense, it is now a concealed weapon. At least that’s what the caselaw says.

So I have a 'pocket knife' that I use all day at work, but if I use it to defend myself after work (say in a walmart parking lot) it's now "illegal"

Not at all. But let’s say that the police stop in frisk you and ask you why you are caring a knife. If you respond I use it to cut things all day long at work, you are good. If you say I’m carrying it for self-defense, you are carrying a concealed weapon. Like I said, it’s all about intent. You can pick up a chair and defend yourself. That doesn’t mean the chair is a weapon until you employ it as such.

I don't carry knives, I only carry seatbelt cutters in case a life needs to be saved in an accident.