Dragged out my serger to take a look. My diagram also stops at the loopers.
If you look at the needle, you will see that there is an open area to the right of the needle. Once you have threaded a looper, you just continue the thread up through this opening near the needle, securing the thread in place with the presser foot. The thread does not go through a needle; once it is through the looper you just run it long enough to be secured between the presser foot and the feed dogs. That's it.
Maybe you are over-thinking it? You're almost there!