Doesn't this imply that the crux of the debate can't be construed as Donatist vs. non-Donatist conceptions, as if there is a qualitative distinction between the positions, but rather that it's a quantitative question of how thoroughgoing heresy must be to invalidate a sect's claim to being a "church" and thus their administration of sacraments?
Something is amiss here. The basic premise of the catholic (universal church) position is that the ordinance depends upon the institution of Christ, not the church administering it. As soon as one brings in the status of the church this basic premise is denied and the separatist position of Donatism has become the working principle. If it depends on the church administering it, the requirements for establishing a valid baptism will be self-referential and undermine the catholicity of baptism.