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I don't think that light-like is a crucial assumption; m=0 is just an approximation which does not affect the relevant physics.
I never thought about your question in detail; for me the structure of the matrix element simply followed from Lorentz covariannce which dictates that there are some trivial kinematical factors plus non-trivial functions involving F(x,Q²). The Lorentz structure of the matrix element is valid w/o ever referring to perturbation theory.
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