I want to use these inside a fish tank. Is there an epoxy coating on the Nautica lineta, small land snail, or apple snail shells?

No I used these shells in a shell dweller tank. All I have to say is you have to hold the shells under water and shake them until all trapped air escapes or shells will float

Can you use these for cooking escargot?

No. I mean I guess you could but why? These shells are thick and I'm assuming it would greatly alter the cooking time and temp. Plus they're really small compared to the more traditional shells escargot in which they're typically cooked and served.

Would these work for saltwater hermit crabs in a tank with corals and fish?

The #4 option are perfect for Blue legged and Red legged saltwater hermits.

What size is the opening?

The opening size varies - rather wildly at that. Be careful what they advertise in the product description. I have found that in almost all cases the size that they're claiming the shells or their openings to be will not be what you actually receive.