Meandering thoughts from an amateur:

I am just a hobbiest, not a scientist, so take this input for what it's
worth. I read whatever I can, but my background is art, not biology, alas...
(A big hurrah and thank you from me to all those biologists who take the time
to write books for the general public, not just academic papers. )

About breeding cones in captivity, even if it has been done (I have no idea),
the captive-bred generations might have lost their toxicity. I believe this
is the case with the various poison arrow frogs. According to an article
about blue-ringed octopus that I recently in an aquarium magazine (I don't
have an aquarium, just bought the mag for the octopus story), these octopus
and poison arrow frogs and puffer fish and cone shells have not merely
similiar, but identical poison. This because the poison is not produced by
the creatures in question, but by a symbiotic bacteria that they host. (I
don't know about you, but his blew my mind).

I've read that the dart frogs in captivity have non-toxic offspring.
Something about the conditions or diet isn't being replicated. It stands to
reason that you'd have the same problem with cones. I'm assuming the research
has to do with properties of the poison, so home-reared cones wouldn't be
helpful.

On the flip side, if the venoms are identical, perhaps an animal with a less
precarious population could be used. Maybe some of the puffers are less rare?

We do eat bushels of clams and shrimp (well I don't anymore. The oceans are
so polluted that I don't feel safe eating clams and the by-catch sitution
with shrimp makes them indigestable, i find a clear conscience helps the
gastric juices to flow) but these aren't predators and there are lots more of
them.