I often say that Platonism (my kind, at any rate) privileges the who over the what. This is how I express the recognition of a positivity beyond the negativity of species-being, a positivity that, while it has its fullest expression among the Gods, reaches all the way down, shadowing every essence; thus forms, for example, eidê, have their theurgical 'shadows', the 'tokens' or sunthêmata.
The whatness of something is always eliminable, its elements reassignable; the 'whoness' refers, ontically, to the possibility of that thing having a different whatness, a different essence—of something's differing essentially from itself.
This sounds exotic, but it answers in the first place to a very basic demand from dialectic, namely the ability to assign something a new definition; this, I believe, is the humble roots of henology in elenchus. It also answers to an existential demand, that an agency not be subordinated, in principle, to the history of its previous acts in any other sense than bearing, so to speak, the responsibility for them (recall that aitia, 'cause', is originally blame/responsibility). The traces of action do not immediately determine an agency to repetition, even if semiosis does. As you correctly point out, however, even semiotically there is no iteration without difference; Platonist semiotics formalizes this negative condition, and uses it as an index for agency.
The Platonist thus sees, on every plane, monadic sites of enunciation or novelty, sites we are always picking up in the middle, so to speak. Truth, with respect to such a site, is coherence within the string, on the one hand; on the other, it is the total configuration of the plane on which it is plotted, by virtue of which there is a place for it. If this place is eliminated, say with the coming of a new paradigm, we can either hold onto some thread and 'leap' to a new plane, as new relations of consistency cohere around pieces of the old plane that have broken loose, or stay on the old plane as it reconfigures itself, continuing to engage the players who still 'exist'. (This is the sense, always local, of the 'leap'.)
One cannot 'prove' the continuity of such a monad. Why doesn't this skeptical problematic trouble the Platonist? Of course, it could be said that the Cartesian cogito has its historical roots, via Augustine, in Platonic anti-skeptical arguments; but why isn't this central to Platonism? I don't believe that it is a question of "intellectual intuition"; I think that Platonism is coherentist all the way down when it comes to questions of verification.
Production, however, is a different matter. Platonism is about unit causality, whereas I'm tempted to say pantheism is about causal unity. This is what is most monotheistic about pantheism: skepticism about assigning causality to any agents, rendering all causal relations exterior, and hence immanent to the subject of experience, itself passively constituted in experience. From here, we end up either with Hume, and anonymous constitution, or with Malebranche, or for that matter many Muslim thinkers (not Suhrawardi and the ishraqiyya, however, who operate within a phenomenological paradigm), where monotheism and occasionalism are inseparable. Some of Deleuze's tendencies (his stress on the univocity of being, for instance) lead in this direction, as well.
Deleuze's reading of Spinoza, however, which is, ironically, rather Leibnizian, leads in a different direction; in effect, from ontology to henology, from immanence to a transcendence necessarily plural. It's an ethical calculus of sorts: the more compossible agencies a monad can recognize, the more it trims its total index of disintegration. This calculus can be used to reconstitute the old hierarchies and privileges, up to a point, but only up to the point that alliance passes over into reduction. It is to prevent reductionism having the last word (again, reduction is available, but always as a local strategy) that it is so important to Platonists to occupy the extremity of any series of reductions with a principle absolutely transparent: this is where they direct their effort, while the procedure of the cogito, for example, remains marginal for them, because it is everywhere: he/she/it thinks, therefore he/she/it is. So, one might say, it is simply a universal passive synthesis; but for the Platonist there is an ethical dimension to passive synthesis. Even if I take responsibility for nothing, I am responsible for the recognition, and hence allocation, of agency. Even, that is to say, insofar as I am merely a moment in the self-awareness of a God, I am responsible for the integrity of an entire plane of manifestation; and this can operate anonymously, akin to Rawls' "original position".
So the 'who' of Platonism is at once personal and objective, and the dialogical nature of henology prior for the Platonist to the circular/monological/holistic/differential quality of the linguistic medium. This, at any rate, is where my thinking stands at present on these matters.