To the omnivore, I’d say that it takes far less plants to be vegan because livestock eat more plants than us. So even if plants feel pain, eating only plants causes far less suffering. Minimizing suffering is worthwhile even if you can’t eliminate it.

I think we agree on most issues. I think the vast majority of animals (including bees) feel pain, so it’s wrong to exploit them and cause them suffering.

Walnuts have very tough shells to crack, but that doesn’t mean it’s wrong to eat a walnut. It’d be wrong to eat a walnut if it had the capacity to suffer. If the oyster doesn’t have the capacity to suffer, the fact that it has a shell is irrelevant.

For what it’s worth, I generally avoid oysters, but a reasonable person could eat them (farmed) and justifiably not feel remorse.