It's such a minor part of my DIY mix, that you can completely leave it out. To be honest, it makes no difference if you are concerned about the copper.
Copper is toxic to shrimp at about 0.4 ppm. I don't know what that translates to in %.
I know Dave from Aquagreen, he is very knowledgeable when it comes to anything Aquarium. He is probably right. And 3% might not be enough to be harmful in that one product by itself. However, the issue is that chelated copper can be present in other things ... food, other ferts, dechlorinators, tap water, etc... and it all adds up. Avoid it if you can to give you a bigger buffer.
Leave it out. You don't need to add a micronutrient mix if you are not completely happy with it. It can always be added later, when you find a product you are happy with. Besides, it is only needed in micro amounts, hence the name. You want to be more critical about the "Macro" ingredients, the ones that name up the bulk of the DIY mix.
Unfortunately yes. I bought the calcium sulphate off Aquagreen many years ago, and I still can't use it, because its a pain to dissolve.
I gave you a link to CaSO4 above from Keg King. Try that.
You can replace CaSO4 with Calcium Carbonate if you want to make a mix for Neo Caridina. This will give you the equivalent of Salty Shrimp GH/KH+.