Hello Ml, I saw your discussion of this topic on Ed's blog. Thanks for taking the time to bring it here!

Some brief comments follow:

ML wrote:

(1) Its reliance on the identity of indiscernables (IOI) - why accept this principle as necessary? I've never come across a defense of this principle in Feser's work. Klima above doesn't defend the principle, and neither does Kretzmann in his discussion of the Summa Contra Gentile's arguments for the uniqueness of God.

I would query that it depends on IOI instead of the much weaker Aristotelian tenant of Matter as the principle of individuation. Perhaps we might admit that if one drops this principle and opts for a Haecceity type account then the positive case for uniqueness losses some of its power.

Maybe we can only give a negative proof i.e. that we have good reason to think that the notion of there being two or more of this type of being would lead to in principle unsolvable problems. Many of these focus on identity problems - how could one, even in principle distinguish these two beings.

Others, which I'd guess you're familiar with given your point 5, is that the existence of multiple beings with certain properties e.g. Omnipotence is incompossible e.g. to go with the example I've chosen if an omnipotent being couldn't affect the actions of another then it would not be omnipotent, yet if a being could have its actions curtailed by another then it wouldn't be omnipotent either.  One could object to that last example by appealing to the notion of a pseudo-task as in the Paradox of the Stone (if there's no disposition to be actualised then there's no lack of power). A more difficult one would be Omniscience, which would take us deep into the territories of de se knowledge and Cantorian gripes about propositions.

ML wrote:

(2) Applying the IOI to spaceless, immutable beings - The IOI seems most applicable to mutable beings in space, such as everyday objects. After all, two beings that have identical positions in space are arguably identical, and two beings that change in exactly the same way (same time, same place, etc.) are also arguably identical. But whatever appeal the IOI has seems to disappear when we consider the case of two spaceless, immutable, perfect beings. 

I think this intuition could be turned around. A mutable being in space has dispositions to be in a different state from its twin e.g. it dispositions to melt, flatten or lose its curvature could be in principle be actualized without those same properties of the twin being so. In the case of a being which is actus purus there are no dispositions which could be counter-factually actualized in a way to distinguish it from another of its kind.

ML wrote:

(3) Counterexamples to IOI: Max Black's spheres

There is an easy metaphysical objection to both the IOI and the symmetrical universe thought experiment: property-instances/tropes. Although each property-instances falls under a universal (according to Realists) it in itself is a unique unrepeatable entity.

ML wrote:

(5) Even if IOI is true... - For the sake of argument, suppose the IOI is true. Whatever the difference between two Gods could be, why think it has to be limiting or qualifying or restricting? An option that Klima and Feser don't consider is this: the difference could be *neutral* with respect to its owner's perfection. That is, it neither adds nor subtracts from its owner's perfection. 

Given Divine Simplicity would it even be possible to have a non-brute difference, let alone one that is neutral? Care to give us a potential example here?