The big questions! There's also the consideration that the final motif wasn't all that important, maybe the actual process of sitting/standing there for hours whacking one rock of another was the important thing. Think of all the old dears over here going for hours with rosary beads repeating the same prayers over and again. Just the long monotonous process produces a change in a persons state of mind, and most if not all religons incorporate processes of altering conciousness through dancing, speaking in tongues, meditating for hours in silence, physical pain, sensory deprevation etc. etc.

Maybe they were similar to the penetential cairns that were built up intil recently, people either voluntarily or forced into taking on long, laborious tasks to make amends for sins etc.

Megalithic art does have the extra element of being associated with a physical structure unlike open air rock art, which gives it an extra dimension where it could be tied with the actual architecture and the variety of motifs sometimes reflects this. They could also have had a social role, who could see what and who was in on the 'meanings' and who was excluded.

Your brain can hurt from going over the possibilities!