Idea, Sufficient Reason, Individuation

According to Schopenhauer, a Platonic Idea is independent of both the Principle of Sufficient Reason and the Principle of Individuation.  Now, in #30 of World as Will and Representation, he characterizes the relation between an Idea and any of its instantiations as that of archetype to copy.  However, he does not explain how the archetype-copy relation is not a species of the ground-consequence relation that, according to the Fourfold Root of the Principle of Sufficient Reason, defines that Principle.  Furthermore, according to one prominent traditional interpretation, a Platonic Idea is the cause of its instantiations, which means that not only is it the ground of which they are the consequences, but that it functions as their Principle of Individuation, as well.  Yet, he does not even entertain the possibility of that interpretation.  So, the independence of the Platonic Ideas from either of those Principles is not a thesis that Schopenhauer adequately defends, to the detriment of his system.