[bugbotherer]

Fascinating thread.
Here's a few of my thoughts for the melting pot. When a snail is overwintering they are firmly stuck to a surface using a modified mucus secretion that may be repelant to predators as well as an attractant to other snails. Once it's stuck down the only way to get at the soft bits would be to either go through the glue or drill through the shell. Another thought is that they may be parasitised and then the only way out for a parasite would be to go through the shell (if it's stuck down). There are a number of dipteran snail parasites (who's life cycle is just tad hazy in this respect) that might be considered along with the beetles.