When is the Tenor Falsetto acceptable

there are high tenors whose voices go into the alto range - the late Enrico di Giuseppe was one. There are roles particularly in French opera with tessitura above that of the tenor; take a look at Leopold in La Juive, for instance. 'Head voice' was the quality expected in such roles and at such altitude so that if an Oberlin, Ferrante or di Giuseppe were to undertake such a part, one would be hard pressed to say why we called one a countertenor, another a tenor and the third either tenor or countertenor depending on the role and his production.