Does the species of snake that eats Amphidromous inversus have chiral dimorphism?
I don't know much about this particular species of snake but here are some info that may help.

What do we mean by right-handed snakes?

I doubt that in this context a snake could be ambidextrous. What they call handedness in these snakes (Pereatids) concerns the density and size of teeth on each side of the jaw (pictures from Hoso et al. (2010))



This asymmetry helps them to forage on snails.



Here is how they describe the snake feeding behaviour.


  The Japanese pareatid, P iwasakii, strikes an exposed soft
  part of a crawling snail from behind at a determined leftward angle
  to grasp the ventral shell surface with the upper jaw and the soft
  body by inserting the mandibles into the aperture. The snake then
  extracts the soft body by alternately protracting and retracting the
  asymmetric right and left mandibles.


On the figure below, the individual on the left is sinistral and the individual on the right is dextral. Note by the way that snails of different chirality have a hard time to mate together.



"Right-handed" snakes can easily forage on dextral snails (the most common and ancestral form), while they have a hard time to forage on dextral snails. Hoso et al. (2010) reports a survival probability of 0% for dextral snails and 87% for sinistral snails.

Left-handed snakes

Hoso et al. (2007) report that "left-handed" snakes have never been found in south-east Asia. This is surprising as noted by Hoso et al. (2010). Even in areas where sinistral snails are relatively common, all snakes were "right-handed".

They do not suggest any potential explanation for this lack of "left-handed" snakes but one could speculate that "handedness" in snakes might be under other selection pressures than those related to foraging. It is also possible that intermediate forms are deleterious and that mutation of large enough effect are rare. It is possible that the genetic variance for handedness is just not present. It is possible that even if in some areas there are more sinistral species of snails than dextral species, the abundance of dextral is still higher and therefore, there is no selection for switching to being right-handed.