That's how big mine was. Several clutches of eggs didn't change her shell, so I'm not sure...do you mean cracks in it, or discoloration, or peeling?
It was pretty funny to watch her chug along into spots where she used to fit, only to find that >bonk-bonk< she'd grown too big. Once she shoved her way into an arch and got stuck. I watched her figure it out for ten minutes, turning and feeling her way through it. Sort of like a car that's parked in and has to negotiate a 45-point maneuver to escape. Could her size, bonking into things, be causing marks on her shell?
I didn't have any luck with spinach or carrots either, but I hit a winner with zucchini slices. Everyone goes nuts for it (all snails, not the betta, he's totally addicted to his color pellets). Cucumber is a close second. Seems odd to blanch cucumber since we eat them uncooked, but I have to trust that the snails know what they like. Both float, so I skewer the slice on a fork and set it tongs-down to keep it directly where they'll find it easily. I have three nerites - the cleaning crew - and the magenta; at times I've seen everyone munching on one zucchini slice at the same time. They'll eat it until there's not a scrap left. I've read that vegetables should be left in for no more than 12 hours; some say 24 hrs. I've found that they're not interested until around the 24th hour, then they start in on it. As though it's not good until a bit of rotting has set in? Anyway, I guess we all have to figure out our own version of the rules based on that the tank population shows us.
I've also given them small flowerettes of broccoli, which they love, but it gets too messy. Some prefer the stem, some the little flowers. One the flowers are being chewed they start to dissolve and make a cloudy stinky mess. Sorry guys, no more broccoli for you.