In order to keep things from being too complicated, I think smithing would need to be always available, but the individual ores would need the tech to make them. (brings in the concept of the bronze age, heh?)
That'd be interesting, but I think shifting
everything towards the inventing system would be a mistake.
But to have different civs during worldgen latch onto local materials and really master the ability to use it would be great. Instead of having various "levels" of workshops though, I think it would be better to put the material dependence on the dwarves. You find iron and can only make basic iron crafts, then you get an immigrant who heard of your hematite find and moves to your fort. Now you can make better quality iron weapons and armor. Also, your material-skill-less dwarves would gradually begin to learn how to use the materials too, so you don't have to depend on an immigrant showing up.
I think that most of all, with both the invention and the material system, I would like to see actual apprenticeship situations. Workshops should be able to hold two dwarves. The dwarf with the lower skill level is essentially the novice, the one who goes to fetch the material or carries it to the stockpile from the workshop. The other dwarf, the master, stays in the workshop and works on materials. When the two are in the shop at the same time, the novice's skill level begins to go up.
When the master leave the shop, the novice takes over and works on materials. A master/teacher/instructor/whateveryouwanttocallit would be able to teach the novice/student/apprentice/etc up to his/her own skill level, but no higher. A dwarf without a teacher can learn skills, but at a MUCH slower rate (slower even then the system we have now) while with a teacher the dwarfs learning would be accelerated (at the speed we have now, which is a pretty good speed).
That way you have the dwarf with the knowledge of iron teaching the other dwarf a knowledge of iron. Longsword dwarf is teaching novice dwarf skills in that, etc.