Is this an ok understanding of Dharmakirti's vināśitvānumāna? The point is that such moment by moment destruction is spontaneous
  (ākasmika) and is the uncaused real nature of things, because it
  cannot be an effect of any cause. The effect of such a cause, i.e.,
  the absence of the entity, would have to be a type of non-being
  (abhāva), and non-being is unreal.
  
  A key underlying principle of the vināśitvānumāna is that negative
  facts, such as absences, are not part of the ultimate furniture of the
  world, but are just fictional conceptual constructions, as they are
  devoid of causal powers. Equally, a fiction lacking causal powers is
  not the effect of something else. While it is obviously impossible to
  deny that hammers smash pots, the absence (abhāva) of the pot, i.e.,
  the non-existent pot, is not an effect, just as other non-existent
  things (abhāva), like horns of rabbits, are not effects of anything
  either. Hammers and the like are thus not actually causes of the pot's
  absence but of it turning into potsherds. That idea is perhaps
  defensible, in that arguably the mere absence of something—a purely
  negative fact—might be less real and less efficacious than the
  presence of other things.[16] Nonetheless, the rest of the argument
  looks to consist in a number of non-sequiturs going from that
  difference in efficacy and reality between absences and presences to
  the idea that perishing is somehow the real nature of things, that it
  must be intrinsic to them, and that therefore things must perish
  spontaneously moment after moment. Let's grant the Buddhist view that
  the perishing of x is the real property of changing into a new thing,
  and not just x becoming absent. If it is accepted that hammer blows do
  change pots into potsherds, then why couldn't someone skeptical about
  the Buddhist's arguments just take that as the model of how things
  perish when they do? It does not follow from that model of perishing
  that a pot could not endure for quite a while.


Is it a non sequitur? 

If there are no absences then is nothing excluded from the causal context of an effect? If so, then couldn't we just conclude that causation is beyond conceptualization, so that any cause (the hammer) of e.g. destruction (the broken pot) is also not the cause, and the effect of destruction must be "the real nature of things", so invariant?

Is that a good reconstruction of the argument, or have I misunderstood?
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.