I've had lots of snails and they die naturally in different ways. Some hide in their shells and die of old age, some flop out of their shells and die. But unlaid eggs definitely do not build up in side a snail and "force out the stomach". Snails will plop out eggs even without soil or peat, if they need to. It is just that they prefer to bury eggs in soil and will mate more often if there is soil available. Most vets have very little knowledge of snails - they deal mostly with cats, dogs and Guinea pigs (or horses and farm animals if they are rural vets).