Kudos for a good comprehensive opening post. Let's see if I manage to contribute somewhat.

In my view, there are irreconcilable differences between Aristotle and Plato, but it's not very easy to determine that they are irreconcilable.

The differences of style are easy to determine though. Plato wrote dialogues, Aristotle wrote treatises. Therefore Aristotle's thought looks systematic, while Plato is subject to interpretation. One can always question if this or that view by Plato is put forth in such a manner that Plato himself endorses it or not. In my view, Plato precisely intended it this way, so that the reader would think along and question the theories put forth. Plato did not teach what to think, but how to think. Aristotle, on the other hand, taught what to think. This is the difference of style.

Aristotle's Metaphysics contains criticism of a kind of theory of forms where forms are conceived as apparently Euclidian solids in the "realm of forms". It's not clear to me if this is a criticism of Plato's theory of forms, even though many interpret it this way. Aristotle's Metaphysics mentions Plato, but on my reading always respectfully, never in connection with the criticised theory of forms. Moreover, the criticism is not against all theories of forms, because Aristotle himself holds to a (different) theory of forms (hylemorphic formal causes).

In my view, Plato's own theory of view would not be open to such criticism anyway. Instead of Euclidian solids, Plato's forms are holographic or ephemeral ideas in one single (ontological monist) substance, while the bodies of this world are condensations (shadows) of those forms. Basically, I take Plotinian interpretation of Plato to be correct.

Flaccus wrote:

Isn't it a major position of the Neoplatonists that there is actually a harmony to be found in Plato and Aristotle?  

The Suda indicates that Porphyry produced six books, On Plato and Aristotle Being Adherents of the Same School (Suda [color=#252525]Π 2098 8-9).
 

There's an important difference between Plotinus and every other Neoplatonist. I see no way to reconcile the view of Plotinus with Aristotle. Plotinus is quite emphatically spiritual monist, and forms and matter are emanations from the single substance or essence, whereas for Aristotle, Forms exist as real, probably immutable "natural kinds". For Aristotle it makes sense to talk about things like "horseness" as a true Form, whereas for Plotinus, if there are Forms (plural) at all, they are subdivisions or distinctions of spirit, but spirit is essentially indivisible (as per divine simplicity), so those distinctions are passing in temporal terms, and passing things are ultimately unreal.

Porphyry was an important scribe/editor/disciple of Plotinus, so for example the all-important Enneads have reached us via Porphyry's hand, but Porphyry's own thinking is of a different quality. I have not read Porphyry, but I would dismiss (his) attempts to reconcile Aristotle and Plato as misguided.