Inverts ,may not be affected by ich but they can carry it.
I notice that you keep bringing this point up, and I wonder why, since it's immaterial to any discussion regarding ich. First off, I've never seen any proof of this. Even if you google it, there are articles here and there that say it's suspected that inverts can carry ich. Nothing that says it's been shown or proven.
Second, there's nothing you can do about it if they are carrying ich. You can't treat them with any ich medications, and if you quarantine them, there's no way to know if they are carrying a hitch hiking ich organism or not. There are no tests available to see if your new snail has an ich hitch hiker or not. And if there was, what are you going to do about it? Nothing. By this same reasoning, you cannot know if your sand or live rock is carrying it. So what? Are you suggesting that every piece of rock, sand and invert that is placed in our tank should be quarantined? If so, what are you going to find out from that? What are you going to wait to see? Since inverts never get ich, you are going to quarantine your new shrimp and snails and wait for...what? Are you suggesting that every single piece of rock, sand and invert is treated for ich? If so, good luck keeping a reef tank.
I'm just very very confused as to why you keep bringing this point up and where you are getting your information from to back up your statements...
The truth is, instead of being so concerned about one ich organism riding in on the shell of your snail, if you keep up your good water quality and keep your fish healthy, IT WON'T MATTER!!! Your fish will be fine! You'd be hard pressed to find anyone that has completely 100% eradicated ich from their tank. The reason people don't see it in their tanks is because their fish don't have outbreaks.