GEddie
Can you provide sources/references so I may better understand your view that extension is prior to corporeality (probably a clearer word than “body” in this discussion.) There are various schools of thought on this matter from memory.
Another related angle that I have not mentioned in this discussion is the distinction between the “substance” of a thing and its “properties” (“accidents” is the traditional word but won’t make much sense to readers).
(This is directly related to the church philosophy often underlying explanations of Transubstantiation as indirectly mentioned above by another reader.)
The first property of matter is “quantity” which admits of various sub-degrees in a hierachical order. Classic “spacial extension” (i.e. as defined/understood by Descartes,Newton and 21st century men and women) would be considered under this property. And spatial extension is nowhere near the top in this hierachy of the realisation of “quantity.”
Thus, in logical theory at least, a substance can be quantified without formal spacial extension and can therefore possess corporeality because corporeality need not rely on spatial extension to be validly defined in other, higher and more abstract quantitative terms. There is a distinction between continuous quantity and discrete quantity and spatial extension.
“Corporeal substance, as such, is not quantitatively divisible. When actuated by quantity it becomes so; but is not yet spatially displayed. The accident is thus distinguished by Scholastics from the further accident of formal extension which is complementary to it, and by which the parts, already rendered distinct by quantity, are localized in space.”
This link makes interesting reading - but it will do your head in even with formal philosophic training:
newadvent.org/cathen/12591a.htm