Is this an ok understanding of Dharmakirti's vināśitvānumāna?
I'm not familiar with Dharmakirti, but from a cursory skim of the argument, it seems he is using typical Nagarjunian reductio-ad-absurdum to yet again show that all (spatially and temporally) discrete things are mental imputations.

It seems he is saying, if things were real, then they would have to perish for real, but if things perished for real, then the effect of the cause that would lead to such perishing would have to be something completely absent. Now, since absence is not real in any sense other than as a mental abstraction, the perishing cannot be anything other than an abstraction. Since perishing is an abstraction, then temporally discrete things must be imputations, hence the ontological momentariness which is just another facet of Emptiness.