What you call a sentence for which presuppositions fail, such as "John's wife has red hair" when John is unmarried, is a matter of definition. The presupposition failed and so the supposition cannot be evaluated, and there is no further fact of the matter. It could be false, or meaningless, or even ungrammatical depending on how you define terms, and arguing over those definitions is quite unilluminating.

Hmmm. I agree that arguing about those definitions is probably fairly pointless. But I also tend to agree with Russell that working out the consequences (advantages and disadvantages) of each of those possible definitions is a very suitable occupation for an intelligent man stuck in prison. :)