In theory, could you create weird structures or even alloys [with Forgery]?
Two scenarios: using Forging how you might use Soulcasting, such as turning something into wood, bending and carving it to make a new shape, then breaking the seal so it turns back into the original material without changing shape.
Second: Forging one metal into another, and forming an alloy to reduce operation costs. For example, tungsten has a melting point of about 3.5k C, but it's useful as part of a steel alloy for certain applications. Could you Forge it into, say, zinc, which has a melting point of about 500 C, create the steel, then turn it back into tungsten? Presumably the investiture would adjust the molecular structure so it acts as if it has been alloyed with tungsten originally too, otherwise the same process happening with food could be deadly.
Yes, this is in theory similar to Soulcasting. The difference is that a Forged object, upon "Forgetting" the rewriting to its spiritual nature is going to try to snap back and match what it "thinks" it should be like--which isn't going to lead to as much stability as Soulcasting, where the actual soul is changed. The object is going to try to get back to the way it "should" be, with varying results.
The reason the Lord Ruler aged hyper-quickly is related to this as well.