I am impressed by the good number of 'tunnel' metaphors in English. It works for my present situation. I now have a tunnel.
I finally took my adviser's advice and tried to write out the first line of each chapter. It was useful in many ways. This made me reconsider where the chapters begin and end but also what each one will contribute to the whole. I can't say that I seceded in writing a single, good opening line for each chapter that encapsulated the argument. Many of my "opening lines" are rambling paragraphs.
Nonetheless, I can see the whole dissertation stretching out before me. It now has shape, form, and substance! One might call it a tunnel. It has enough substance that I know what remaining sources I need to see in order to write each chapter. I can also see how the various arguments twist and turn as they run through the story.
I definitely don't see any light at the end of this tunnel, but I do, finally, see a tunnel!