What all this means is that the passions tell us what we want to do and get us moving to do it. Reason alone is insufficient. There’s nothing inherently bad about death, for example. We could live or die. Neither of these states are contrary to reason. But we don’t like dying, and so we use reason, in combination with our desire to live, to find food, cook something, etc. Reason here is subservient to the desires, and as it is in this, Hume claims it is in all things.
I would have to brush up on my Hume, but from what I can remember about History of Philosophy, Hume was big on our experience of the World influencing and making up our concepts. Our “passions” I would think, are caused by our experiences of the world that influence base desires and aspirations. Reason comes into play when we deliberate about how to achieve those desires. In contrast, Kant thought that our experiences were only one side of a coin–our concepts were essential in leading or guiding our experiences.
I’m not sure where you get the “eating metal” analogy from. I haven’t been able to find it with my admittedly quick googling.
But, overall, for Hume, there is no such thing as passion overriding reason. The passions are always above reason. Reason is the servant of the passions, telling you how to do things, not deciding what to do.
So, the idea of things being “unreasonable” doesn’t exist for Hume. We might say, “It’s unreasonable to require everyone always wear purple hats,” but this isn’t an act of reason, according to Hume, and so it really isn’t unreasonable. At its core, it is our emotions that are objecting to such a requirement.
You could argue that instrumentalism follows from this “subservience” of reason, since there’s nothing reason can tell us that allows us to develop moral rules. If scratching my finger is the same as the destruction of the world, by what right can I object to your attempts to destroy the world? But such a view either ignores or denies the emotions a role in forming moral rules. This is part of why it’s difficult to describe Hume as an instrumentalist.
ETA: If we do allow the emotions some role in the formation of moral rules, then these emotional motivations must be their very foundation. This means that to make a moral rule, we apply the use of reason to these “moral emotions,” and, as a result, we create rules that we believe are useful for us to attain our other goals. This makes morals into a question of expediency, without the claim to being Morally Right that such claims usually involve. Hume doesn’t really go this far, but it’s where the general train of thought tends to lead.