I'll give it a shot. From what I know cuttle bone raises kH but not gH.
Also, pH is raised or rather prevented to go lower by kH. If you raise kH it will have an impact on pH.
gH will not raise pH there is no connection between these two. kH can be raised independently of gH by adding baking soda.
gH can be raised independently of kH by adding Calcium Chloride, Calcium Sulfate and Magnesium Sulfate
Your snails are suffering because the water drains minerals from their shell as it is, most probably, too acid for them. Snails can get their minerals from what they eat and the water itself if it has the right parameters which yours doesn't. On a side note, plants benefit from this water as acid, low kH water is better for some of them.
Clay helps but it doesn't have much Calcium in it. Gipsum (aka Calcium Sulfate) is a good material if you don't want to spend money on commercial products.
I'm using RO water in all my tanks and one of them does not receive anything than fish food but it's full of snails that do pretty good.