Fundamental Molinist conditionals of free will about non-existent agents are brutish: they are not grounded in other propositions, nor made true by a truthmaker, lack of a falsemaker and/or the obtaining of properties/relations between entities.
Now, suppose as seems plausible to me that there are precisely two kinds of explanation: constitutive-style and causal-style explanations. Constitutive-style explanations explain a truth by explaining how the truth is grounded: the knife is hot because its molecules have high kinetic energy. Causal-style explanations explain a truth by giving non-grounding conditions that nonetheless in a mysterious but familiar causal or at least causal-like give rise to the holding of the truth.
Now, brutish truths have no constitutive-style explanations. For the constitutive-style explanation involves the describing of a grounding. But brutish truths also have no causal-style explanations. For causal-style explanations involves the describing of causal-style relations between the aspects of the world (in the concrete sense) that ground the explanandum and explanans. (In fact, for this reason, brutish truths not only lack causal-style explanations but are not causal-style explanations for anything else.) So, brutish truths have no explanations.
But if there are true fundamental Molinist conditionals of free will about non-existent agents, there will also be ones that have explanations. For, some, maybe all, free actions can be explained in terms of the reasons the agent had. Thus, Curley accepts the bribe because he wants to be richer. Granted, this is a non-necessitating explanation–that Curley wants to be richer does not entail that he accepts the bribe. But that’s still an explanation, and one of causal-type. And exactly parallel explanations can be given for Molinist conditionals. Thus, Curley would have accepted the bribe in circumstances C because circumstances C includes his wanting to be richer. And presumably this kind of explanation would have held even had Curley never existed, and presumably if Molinism is true, there are such explanations for true conditionals about actually non-existent agents. Thus some fundamental Molinist conditionals of free will about non-existent agents can be explained. But this contradicts their brutishness.
Moreover, presumably some fundamental true Molinist conditionals of free will about non-existent agents explain God’s creative inactions. Thus, perhaps, God did not create Badolf Bitler, because Bitler would have been so much worse than Hitler. But these conditionals do not provide a constitutive-style explanation for such actions. So they provide a causal-style explanation. But they can’t do that, because they’re brutish.
The same argument goes against Merricks-style presentism on which fundamental truths about the past are brutish. But many, perhaps all, fundamental truths about the past are explained by other fundamental truths about the past.
Why was I all the time talking about fundamental truths? Because both in Molinism and Merricks-style presentism, there are some grounded truths. For instance, that Spock would have poured water into a beaker in C might be grounded in the truth that Spock would have poured H2O into a beaker in C, so some Molinist conditionals about non-existent entities have a grounding. Likewise, that the dinosaurs drank water is grounded in the truth that dinosaurs drank H2O.
I think the weakest part of my argument is my insistence that there is non-constitutive explanation of brutish truths. One might say that in addition to causal-type explanations, there are counterfactual-causal and past-causal explanations. Thus, if C caused E, then the proposition that E occurred is explained by C’s having caused E. And likewise that Curley would have accepted the bribe is explained by the fact that C would have probabilified Curley’s acceptance.
Nonetheless, while this answer is available to the Molinist or to Merricks, it is an additional cost to the theory. For we avoid multiplying types of explanations.