Given theism, I suspect there is no answer to van Inwagen’s Special Composition Question, which asks for non-mereological necessary and sufficient conditions for a plurality of things to compose a whole.
The basic idea is this. Proper parthood is a real relation. God can simply insert or delete new instances of a real relation into the world as long as he does not violate the logic of this relation. Then imagine that God simply deletes all instances of proper parthood from this world. The logic of proper parthood is not violated by this. But we cannot non-mereologically describe the difference between our world and the resulting mereologically nihilistic world, and yet our world is one where (I assume) there is non-trivial composition, and the modified world is one where there isn’t any. Hence, we cannot give non-mereological necessary and sufficient conditions for a plurality to compose.
One might object that the whole depends on the parts (a similar argument can be given if the parts depend on the whole, with the same response), and so God cannot delete the parthood relation between me and my parts, as that will make my parts disappear from reality. But as Aquinas argues, what creatures can do, God can do it without help from creatures. So if my parts can keep me in existence, God can keep me in existence without any reliance on these parts.
Here is a theistic argument directly aimed at van Inwagen’s organicism. Any creature that exists must be sustained by God. God could keep on sustaining my parts without sustaining me, so I drop out of existence. It would still be true that my former parts are engaging in the kind of activity that define life for van Inwagen, but they would no longer compose a whole.