I know there are people out there who throw in human supplements with success, but I've never been willing to place that much trust in those things. There are also people who try human supplements and complain it clouds the water because they dissolve too fast and I don't know which ones are guilty of that.
There isn't really a best type of block as far as the weekend feeders go. Thicker ones will last longer; that's about all there is to it, so just look for the bet bang per buck in that respect. It's also possible to make your own but I've never done it.
Oyster shell doesn't really do much. Neither do any of the older methods for accomplishing this like cuttlebone. Hard things like eggshell and oyster shell are difficult for snails to chew on if they are even interested in tasting it. Cuttlebone is just risky since you can get bad ones that foul the water straight away from not having been prepared properly. Oyster shell and eggshell don't run that risk but just aren't as effective IME, oyster shell being least so except for functioning as an aragonite buffer for the water.
Keep in mind that shell deformities likely won't be affected at all by CaCO3 content. The main impacted things will be healing of fully eroded areas and ability to put down harder shell faster (so less prone to injury). If the shell has ended up funny shaped over time in a developmental sort of way, that's going to be down to how the soft tissue is depositing things regardless of how thick the shell is. If instead you mean something like fissures (which are the result of an injury, not a deformity as is spread around on the web far too much), then you may see some improvement if they are being caused by shells that are generally weak, but existing fissures might not show a lot of improvement if they're already quite sever.