nails on a chalkboard is a fixed statement. The original meaning refers to the unbearable and annoying sound made by scratching on the blackboard with fingernails or hard objects. Here you can understand it to mean "on pins and needles", "can't stand it", "crazy", "go crazy", etc.

Why is sometimes just a modal particle in spoken language, with no fixed meaning.

Did is used for emphasis. For example, I did expect to give you a hand, but I was too busy.