But if what she does in her head governs her actions, and her actions abstract over supposedly incommensurate needs to produce (at least when demanded) a unified sense of the relative value of X and Y, why is there a problem?

How is knowing that you can barter meat for milk to quench thirst any different in kind from knowing you can drink milk to quench thirst? How is knowing that you can work as a bricklayer to earn bigger numbers at a company that they will trade you for paper that you can exchange for olive oil that you can trade to your local farmer for milk to quench thirst any different from that? It's just a longer chain. The need is to not be thirsty, not to have milk. Milk is just a means to an end.

That something has utility for a purpose does not seem to depend on how long the chain is to actually achieve that utility, as long as the chain is reasonably reliable.

It seems like the thrust of your criticism is a refusal to allow people to do the kinds of things that they do easily all the time.