Not sure if these 2 Id dependencies can be extracted from each other. Even if both dependencies - area or "Pelgrom", and offset voltage (variation) - are somehow correlated because both depend on similar physical parameter variations over area effects (gate-oxide thickness variations, mobility variations, and depletion, surface-state and threshold-implant charge variations), they are probably - to a good part - just different representations of the same physical reasons which lead to local area mismatch, s. e.g. David M. Binkley "Tradeoffs and Optimization in Analog CMOS Design", chap. 3.11 MISMATCH, p. 233 ff.
The above given equation ΔId/Id = Δ(W/L) / (W/L) - 2*ΔVth/(Vgs-Vth) could only be valid in strong inversion (because of the Id dependency on (Vgs-Vth)²); in moderate and weak inversion (Veff = Vgs-Vth ≈ 0 or ≤ 0) the second term would cancel the area dependency and would lead to an unrealistic result.