Sarah,
 
Well, I favor the used-as-food theory.  By (archaeological) standards here, 5
cm. is a decent sized shell.   Conus meat is hard to remove so such a hole
would be understandable.  But, I have also found Conus dead on the beach with
similar holes bashed in the side.  I have looked at many hundreds of thousands
of shells from middens in Ecuador, mostly used as food and mostly small
gastropods (1-5 cm.; Columbella, Tegula, Cantharus, and various limpet
species, and so on).  In fact, the numerically dominant shell in the middens of
the central coast of Ecuador is Olivella semistriata, accounting for about 40%
of the total number of identifiable shells.  Very few of these were used for beads
(<1%) and so we assume they were eaten, perhaps in the form of broth.  I have
made some and it is quite tasty.
 
As for using cone poison on arrows or darts, the scarcity of points in the
excavated material is probably not significant.  Bone, wood and bamboo were
often used for arrow points (and still are today by such groups as the
Yanomami) and these materials often do not survive in archaeological deposits.
Poison arrows are usually made of such materials because they are designed
to break off in the wound and thus ensure an adequate delivery of the poison.
The poison arrow points I have seen were made of hard wood and were deeply
scored around in order to break off.  The poison was smeared into the grooves.
 
kate