I am the AP coordinator at a small school. Can I assign myself as the proctor? If I assign someone else as proctor, can I log in to Test Day Toolkit to oversee the administration and help the proctor if necessary?

Once you complete the Test Day Toolkit access process, you will be able to set up each exam administration (create rooms, add staff, assign staff as proctors). Yes, as the AP coordinator you can assign yourself as the proctor to a room that you create. And yes, you can access any/all rooms in each exam administration.