The shell may be falling apart because there isn't enough calcium in the water. The sell shouldn't/doesn't do that because of genes. The easiest thing is to go to Walmart and pick up a 98 cent cuttlebone (in the bird section) and break off a piece and throw it in the tank. It'll slowly dissolve the calcium into the water. If you can't get one at Walmart, you can get them just as easily at a pet store. I like the Walmart ones because they don't have any added salt- always check the back of the package!! If in doubt...don't buy it! It should just be plain cuttlebone. And it won't harm your fish, cycle or tank- it'll float for a few days and then sink. Just take it out when it starts to get nasty and really slimy and add in a new piece.
Most snails are just labeled as "mystery snails" when they are really Apple Snails. Apple Snails are more of a temperate water species- they will hibernate when the temperature gets warm and that's why they don't ever seem too energetic/active. At least that's what I've read anyway. But they are pretty cool and most fish just leave them alone. If they get picked on too much they can get stressed out and die (which is why mine died, at least I'm pretty sure). Just google "apple snail care" :3
And don't feel bad!! Every one has their "little kid fish horror stories". Mine is I kept a platy and 2 cories in a goldfish bowl for a whole summer...a long time ago but I'm sure the poor things suffered!! Thankfully I know better!!