First, I need to comment again on the translation of 'being' by 'substance' in Aristotle, which Joe Sachs criticizes
here. Sachs says in reference to this mis-translation 'It is no wonder that the Metaphysics ceased to have any influence on living thinking: its heart had been cut out of it by its friends'.
So imagine if the passage you quoted above put 'being' in the place of 'substance'. It is not entirely accurate, but I think it conveys something which has been lost in the usual discussion of 'substance':
But the universal too seems to some people to be most of all a cause, and the universal most of all a starting-point. So let us turn to that too. For it seems impossible for any of the things said [of something] universally to be substance [a] being. For first the substance being of each thing is special to it, in that it does not belong to anything else. A universal, by contrast, is something common, since that thing is said to be a universal which naturally belongs to many things. Of which, then, will it be the substance being? For it is either the substance being of none or of all. And it cannot be the substance being of all. — Metaphysics, 1038b9, translated by CDC Reeve
I think the discussion of substance tends to slant the discussion, because it's natural to reify substance as something objectively existent (or more likely non-existent) and that this is at the basis of the difference between the Platonic and Aristotelian doctrine of forms.
I believe that Plato's doctrine of ideas requires an understanding that the 'ideas' or 'forms' are real in a different sense to the reality of phenomena. Betrand Russell says that universals don't
exist in the sense that horses, men, tables and chairs do, but that they're nevertheless real - they 'subsist'.
Suppose, for example, that we are thinking of whiteness. Then in one sense it may be said that whiteness is 'in our mind'. ... In the strict sense, it is not whiteness that is in our mind, but the act of thinking of whiteness. The connected ambiguity in the word 'idea', which we noted at the same time, also causes confusion here. In one sense of this word, namely the sense in which it denotes the object of an act of thought, whiteness is an 'idea'. Hence, if the ambiguity is not guarded against, we may come to think that whiteness is an 'idea' in the other sense, i.e. an act of thought; and thus we come to think that whiteness is mental. But in so thinking, we rob it of its essential quality of universality. One man's act of thought is necessarily a different thing from another man's; one man's act of thought at one time is necessarily a different thing from the same man's act of thought at another time. Hence, if whiteness were the thought as opposed to its object, no two different men could think of it, and no one man could think of it twice. That which many different thoughts of whiteness have in common is their object, and this object is different from all of them. Thus universals are not thoughts, though when known they are the objects of thoughts. — Betrand Russell
I've bolded the significant point, which I think resolves many of these issues. So the 'idea of a man' is just that - but it doesn't exist, not in some 'ethereal realm' or 'Platonic heaven' - not that Plato himself is clear about that, but it became manifest in later (ancient and medieval) philosophy. I think the key idea is that of the intelligible object - something which is real, but only perceptible by reason, not by the senses. The idea or form is what is manifested in the physical form of man. Hence
the simile in this post.
And that - realism regarding universals - is what was lost from the Western tradition with the ascendancy of nominalism over scholastic realism. That's why there can't be any conception that universals exist in a different sense to particulars - because that is an aspect of the conceptual space that is no longer available to us (cf. dfpolis 'post-Cartesian conceptual space')
Anyway, carry on.
//I should add that it's much easier to concieve of an idea of a form as 'the being of all' than it is as 'the substance of all' i.e. the individual is an instantiation of a singular idea. Every man exemplifies 'the idea of man'. I don't see how this presents great conceptual difficulties.