But as you wrote, two things can be really distinct but still inseparable. And this is the case with creatures, in the sense that their essence and existence start to exist together, and are annihilated together. You can't have a created existence without the created essence, and vice versa. Therefore, I don't see how you would argue from the real distinction to contingency (since this argument seems to presuppose the separability of the essence from its existence). It seems therefore that the argument from contingency to the real distinction, AND the argument from the real distinction to contingency are both invalid. Both ways are shut, but they were made by those who are Thomists, and the Thomists insist to keep them.
I don't think that's right. There are, of course, other arguments for the real distinction (say, through conceptual & ontological priority & act-potency analysis). Moreover, one does not need separability to affirm contingency. Rather, one can simply argue that the existence element of something depends upon the essence of that something for its individuation, whereas the essence element depends upon the existence element for its actuality. (One being prior with respective to conceivability, the other being prior with respect to actuality). From there, the argument can be made that this relation of mutual dependence between two distinct metaphysical co-principles demands itself signals contingency and demands an extrinsic unifier. There is much that argument, but the path is definitely not so clearly shut as you seem to think.
Sure, if you suppose that there is potency in every union of existence and essence, then you can argue to contingency. However, the question is this : Why would every union of existence and essence imply potency ? Typically, we say that a composite object implies potency because the parts of that object are either separable or contingent (or both). But now let's imagine an existence and an essence that are necessarily united, and that exist necessarily. In that case, although this existence is still dependent in some sense on that essence, and although that essence is still in some sense dependent on that existence, there are inseparable and necessary, and therefore I don't see a reason to suppose that they imply potency. Dependency (including mutual dependency) doesn't necessarily imply potency. In the Trinity, the Son depends on the Father, but there is no potency involved. Furthermore, there is no need to appeal to an extrinsic unifier in the case of the necessary union of essence and existence I just imagined, since a whole to part grounding could also be a thing (e.g. a whole organism is typically viewed as the ground of its parts like its heart or its kidney). In other words, a necessary union of existence and essence could be the ground of its two "parts" (the essence and the existence), without needing to posit an extrinsic unifier. Or the existence could still be the ground of union with its essence (in a similar way, the Father is considered as the Unifier of the Trinity, even if the Father is Himself a Member of the Trinity)
In short : If we don't presuppose contingency and separability from the get-go, it seems there is no reason to suppose that an union (and therefore a real distinction) of essence and existence should necessarily imply potency (since we don't see a priori a contradiction in the idea that such union could be necessary and eternal). And therefore I don't see how we could argue from a mere real distinction to contingency.
But as you wrote, two things can be really distinct but still inseparable. And this is the case with creatures, in the sense that their essence and existence start to exist together, and are annihilated together. You can't have a created existence without the created essence, and vice versa. Therefore, I don't see how you would argue from the real distinction to contingency (since this argument seems to presuppose the separability of the essence from its existence). It seems therefore that the argument from contingency to the real distinction, AND the argument from the real distinction to contingency are both invalid. Both ways are shut, but they were made by those who are Thomists, and the Thomists insist to keep them.
I don't think that's right. There are, of course, other arguments for the real distinction (say, through conceptual & ontological priority & act-potency analysis). Moreover, one does not need separability to affirm contingency. Rather, one can simply argue that the existence element of something depends upon the essence of that something for its individuation, whereas the essence element depends upon the existence element for its actuality. (One being prior with respective to conceivability, the other being prior with respect to actuality). From there, the argument can be made that this relation of mutual dependence between two distinct metaphysical co-principles demands itself signals contingency and demands an extrinsic unifier. There is much that argument, but the path is definitely not so clearly shut as you seem to think.
Sure, if you suppose that there is potency in every union of existence and essence, then you can argue to contingency. However, the question is this : Why would every union of existence and essence imply potency ? Typically, we say that a composite object implies potency because the parts of that object are either separable or contingent (or both). But now let's imagine an existence and an essence that are necessarily united, and that exist necessarily. In that case, although this existence is still dependent in some sense on that essence, and although that essence is still in some sense dependent on that existence, there are inseparable and necessary, and therefore I don't see a reason to suppose that they imply potency. Dependency (including mutual dependency) doesn't necessarily imply potency. In the Trinity, the Son depends on the Father, but there is no potency involved. Furthermore, there is no need to appeal to an extrinsic unifier in the case of the necessary union of essence and existence I just imagined, since a whole to part grounding could also be a thing (e.g. a whole organism is typically viewed as the ground of its parts like its heart or its kidney). In other words, a necessary union of existence and essence could be the ground of its two "parts" (the essence and the existence), without needing to posit an extrinsic unifier. Or the existence could still be the ground of union with its essence (in a similar way, the Father is considered as the Unifier of the Trinity, even if the Father is Himself a Member of the Trinity)
In short : If we don't presuppose contingency and separability from the get-go, it seems there is no reason to suppose that an union (and therefore a real distinction) of essence and existence should necessarily imply potency (since we don't see a priori a contradiction in the idea that such union could be necessary and eternal). And therefore I don't see how we could argue from a mere real distinction to contingency.