Keep central, state, manufacturer and dealer discounts separate. A dealer discount is not automatically a government subsidy, and two schemes may not be stackable. Calculate affordability from the price you are contractually required to pay today, then treat only a confirmed benefit as a reduction.
Stop condition: if the exact model or purchase date is not eligible on the current official scheme source, repeated complaints to the dealer will not create eligibility. Focus instead on correcting any misleading written sales representation or recovering money promised in the contract.