Custom ore variation and three other questions

Hello! I'm creating a mod (obviously), and I have four questions as to how to do some slightly complicated things. I'm no stranger to coding, so give me your complex custom code answers (but maybe explain what they're doing just so that I don't get confused with anything).

1. I want to create a type of ore that generates with multiple kinds of the ore. As an example, what I would want for this is if I find a vein of the ore, I'll either find the red, green, or blue variant (I have more variants than that, but that should get the idea across). I want to do this because I want all kinds of the ore to be fairly rare, but if I create them separately then they'll spawn 17x more often (as I have 17 different variants of the ore) than if they shared an ore generator.
2. I want to create a new key-bound GUI that you can put only certain items into the slots, similar to armor slots (this, rather than all items or one item).
3. I want the aforementioned key-bound GUI to keep the items put into it to stay from when I close it to when I next open it. I think I have an idea, involving setting a variable to the item's data, but I don't think I can actually do that.
4. For my last problem, I'm creating a new dimension (nether generation), and I want to be able to control the ores that spawn in this dimension. I want to be able to control where they spawn, how often they spawn, and how much can spawn. I think there might be a way to do this by creating custom biomes, I can't remember if you can control ores with biomes but I think so...?

Thank you for your help!