As a general answer i'd say work on the kabbala of your training. By that i mean when you have many questions like this, assign them layers and hierarchy. Connect them to your whole path always. By itself it is a very important part of the great work for all things. The all is one. Though to make it possible that all things are continuous and harmonuous, the Law adjusted them all from top to bottom, all in relationship to every other.
Everything is true, but not everything is useful. How to apply this here? From the highest meditative state you can achieve, go down, while maintenaning the consciousness of the observed thing. When continuity is broken, go back and try again, until it gets continuous.
It is a way, or a part of it, among many others. Everything is true also means everything is optimal for its own purpose and nature. Therefore, exactly ALL WAYS of practice will produce specific OPTIMAL results. So the question is what is optimal for you now. To this assign layers. Then apply again solve and coagula.
Establishing the all in all things, everything is always there. It is a matter of proportion and rythms. This you can start with yin and yang, 3, and 4.
Classify different types of rituals according to different perspectives. Or to schools, or worlds, or whatever you might find useful for your own current actual practice.
A basic way to apply yin and yang here can be for instance to practice a physical version and an astral one. Then work on astral manifestation from physical perspective and vice versa. Focus on the different worlds at play and the respective body layers. Thus you will answer a tremendous amount of this. Questions are infinite. To answer practice. To practice good make it simple. Combine complex and simple. Practice separetely(with emphasis on specific part) then expand. Or balance. All are good.What is sure is if you keep on practicing a couple of rituals seriously you will have good results.
When confused dont mix up things. Just practice a couple of rituals as you choose to follow, and nothing will be wasted, whatever your later preferences might become. The guys are all right, and so are their rituals. Their is no such thing as "right" or "wrong". A ritual is exactly as it is, what could be said more? Is greek more right than hebrew? Just practice as they are and you'll get each layer of answer at each optimal time for you.
As for "metal" i see it as a very holy thing. It is the most classic material for building the cup and the sword, and the ring, and the spear, all great symbols. It can be seen as corresponding to ether, and the great work completed. The philosophers stone. It is also good for talisman. Very powerful. Of course it could be programmed as a black talisman, charged with evil forces.
As most people are of very low level in magick and dont master the projection of their forces, they charge things all the time with evil forces. By repetition, even if it is of low power, it can increase, and thus you can get "evil" objects. Then the object itself may attract independently all sorts of evil beings and so on.