I don't know much about how primitive humans made holes in shells, but the
radula of a gastropod produces a very characteristic hole, larger on the
outer surface of the shell than on the inner surface, with sloping or
slightly concave sides.  The gastropod make a concave or "bowl-shaped"
depression in the shell it is attacking, then continues to enlarge and
deepen that depression until the bottom of the "bowl" breaks through the
inner surface of the shell.  Under magnification (microscope), the sloping
sides of the depression show the grooves or scratches from the radular teeth
of the gastropod, and these scratches generally run more or less vertically
up and down the slope of the hole.  As I said, I don't know what kinds of
tools people might have used, but I would assume it was either a smooth,
sharp tool like an awl, or some kind of rotary rasp.  I doubt that these
tools would produce the fairly regular and uniform impressions that radular
teeth produce, and if a rotary tool did produce similar marks, I would guess
that the scratches might run more or less horizontally, around the hole
rather than up and down its sides.
The fact that most of your shells are drilled near the umbo is probably not
helpful.  Many carnivorous gastropods prefer to attack that part of a
bivalve shell, and I have seen beaches with many dead, "drilled" shells,
virtually every one of them drilled near the umbos.  (I often wondered why
this is so, since that tends to be the thickest part of the bivalve shell.
Any ideas on that, anyone?)  If humans drilled holes in shells in order to
use them as adornment, either hung on a cord around the neck, or stitched
onto clothing, or laced onto other artifacts, then it seems the umbo area
might be the most likely place for them to drill as well(?)
As for octopus holes, I have less experience with them, but my impression is
that they are more like a puncture, and might be difficult to distinguish
from a hole made by a small awl(?)  Hopefully someone with direct experience
can address this.
Regard
Paul M.