If it's all body mechanics then why bother telling her to "drive it through his head"? Why say anything at all? If it's all the work of muscle groups working together why does he need to direct her to perform any mental gymnastics?
Because, as with many things, visualization helps. It helps because we can't mentally single out every muscle in our body (for instance, imagine how unhelpful it would be for an instructor to say, "Use your latissimus dorsae."). Visualization is a useful tool for complex muscular actions. Only in aikido, though, do we consider these visualizations evidence of ki
. In everything else (singing, shot put, flyfishing, whatever), they just are what they are. What makes aikido different?
Also, the idea behind unbendable is to perform it with a relaxed arm and focused intent (coordination of and and body, a.k.a. correct feeling or as Dan posted in #50 "controlled use of the body systems through intent"). When performed correctly unbendable arm can be held for an indefinite period of time. Reliance on muscles alone will eventually lead to the arm bending as the muscles holding the position relax on their own when they become exhausted.
I think you've got a false dichotomy here. You seem to be assuming that one can either (a) use his muscles or (b) relax his arms. I'm not sure that is biologically sound. Even with a relaxed arm, there can still be lot of muscle and bone structure at work.