Underfire,
I think the disagreement is about math, not engineering. No one's disputing that removing the windows makes the skin 25% lighter. And I think we agree that the weight of rest of the fuselage structure is static, or even decreases slightly without windows. But if the skin is 20%, or even 50%, of the fuselage weight, making the skin 25% lighter doesn't make the fuselage 25% lighter.
(I think you agree that even without the windows, a fuselage still has to have things like stringers and bulkheads. None of those go into the 1560 KG in the second formula.)