The greatest consideration should be given to the possibility of graded levels of reason. I mean to say that we have different levels of unconditioned realities. Modes of necessity. So in knowledge of these modes we ought to considered different levels of non intuitive immediate knowledge.
In short: How is a priori synthetic knowledge possible? you could say like the intutionists [Prichard G.E. Moore, Huemer, Bryan Caplan] that it is by reason. They are expanding the areas that reason knows for good cause. They look at Hume's argument that reason has only one function--to `perceive contradictions.They know that that is wrong. But we do have to admit the kind of reasoning we use to advance a mathematical series is different than the other kinds of intellectual perception that they suggest. That is they ignore Kant's arguments, not just Hume.
So Plato could have answered the question how the forms participate with individuals -he could have answered the representation makes the object possible and the object makes the representation possible.