@abob, you and your husband have had quite the ride. He's fortunate to have you at his side. None of the tests or scans are perfect, but they often do fill in one part of the puzzle. They rarely complete the puzzle, which is frustrating.
I am not a clinician, but my understanding is that ctDNA tests (sometimes referred to as a liquid biopsy) are looking for circulating tumor DNA in the blood stream. I've had one of these tests at the time that I was diagnosed. My oncologist has noted that the tumor has to be shedding a certain number of cells, so fairly far along, in order for the DNA to appear in the blood stream. Being that they found 0%, doesn't mean with 100% certainty that the nodule isn't cancer, but it also tells us that it's not shedding cells like crazy, which is good.
Did the oncologist give you an idea of what they believe may be happening? I'm assuming that they can't/haven't biopsied it? How long will you wait for the next PET or CT scan?