What an exciting and worthwhile critical discussion! I believe that the closer structural [phonetic and semantic] affinity between the words "substantive" and "substantial" is largely responsible for the mix-up, especially so in view of the manner in which they are interchanged unknowingly yet increasingly in common usage, leading to the fusion of the fine distinction between them in the social process [with both words as probably derived from the root word substance (?)]. I have witnessed a lot of this across a number of other world languages. My take on this, however, would be that the word "substantive" is temporally [qualitatively, intensively] marked compact space, and the term "substantial" spatially [quantitatively, extensively] constituted vast time, Whereas the former is informed by a sense of "distinctness" [of the compactness of space], the latter by a degree of "togetherness" [of the vastness of time]. It somehow boils down to both variation and emphasis, either of temporality and quality over spatiality and quantity and vice versa, that is, "substantive" over "substantial" and vice versa. Therein, lies the fine distinction between them, either in their separate or combined usage..

[NB. Not that time and space are separable entities. Far from it. For time and space, either as ontological entities -- that is, common medium in which all things are, or as epistemological entities -- that is, their relative arrangement across cultures, are inseparable in reality, as in nature, mind and society. Many thanks.