Clarifications:
Cave paintings, I was referring to the one of the various hands, not the animals. Prehistoric cognitive archaeology is changing its theories all the time over their interpretations of the animals and hands. The hands with various amounts of fingers are the paintings I was referring to. Prehistory, with the exception of hominid evolution, is not really my subject because it cannot answer the questions you pose with any certainty.
I never wanted to imply that the Nazca lines were done by one person, I am sorry if I gave that impression. The experiments I referred to were done with multiple people. We cannot rule out the possibility of a designer leading people. It would be really special if a bunch of people joined together and each shared an equal part in planning the design.
Art does not have to be movable or able to be traded over distances. Architects are enough evidence of that. Land with this art on it can be controlled (reference to the Cambodia/Thailand conflict over a UNESCO World Heritage Site), therefore traded and of value despite their immobility. Perhaps it is their immobility that adds to their value?
End Clarifications.
I wouldn't say you are playing the devil's advocate. This is a necessary part of the scientific procedure. Hypothesis and theories need to be scrutinized by other scholars. I would hate to be talking to myself about all of this or simple having people agree with me without checking the facts. That leads to Daenikin and Hancock.
I will say this about Daenikin though: He may have a theory, but only in the sense that he means speculation, rather than a scientific theory or hypothesis. That point should be emphasized. Too many people think that the vernacular theory and scientific theory mean the same thing.
But I think that we can agree that people and artifacts, including geoglyphs, can and do have multiple functions. A piece of art made for religious reasons to the gods, can also serve as a spiritual path to be walked about (hence appreciated for its design on the ground) or perhaps slept in by an individual as was done for other geoglyphs and was also directions.
You ask about motivation and why like jewelry...
I do not know their economics, but why not have conscripted or hired workers? That is a simple enough motivation for working. Why like the jewelry? The design could (pure speculation) have been chosen by the Big Man/Chief because of his fondness, hence the jewelry of same images (of course we do not know who chose the design). Religious adviser says they need an offering or they need fertility, etc, whatever, a designer leads a team to create the Nazca lines.
Therefore it turns into a religious/artistic/perhaps directional function all at the same time.
I believe it can be agreed upon that it does not need to serve only one function at a time. I look forward to the results, especially being a Leicester grad student