Layered clothing issues with armature, rigging, caging, and texture painting

Hey all.
I’ve been trying my best to learn how to create layered clothing. I’ve followed Roblox’s tutorial exactly over and over but I keep running into issues, issues that I can not find the solution for anywhere online.
I’m using Blender, and I am creating a dress.
Here are some issues I keep running into:

-When rigging, my clothing doesn’t move at all. I have the rig perfectly placed inside my mesh, it is parented with automatic weights correctly added. My weight paint is all deep-blue, no reds or greens.
If I do get the rig to work, like when I move an arm, it drags the bottom half of the dress with it.
I have also tried Roblox’s Auto Skin Transfer, but I have no clue if it worked or not.

-My dress is in two different parts, one white and one black. They are joined together in one big mesh but I am having trouble understanding how to keep the textures on the dress when importing. Here is what it looks like, when I import it, it imports all totally white.

-And lastly, I am confused as to how I need to cage this. If I do use the sculpt tool to cover the mesh, I find that very hard to do. I try going into edit mode and selecting each edge and scaling it just over the top. Either way I try, I get this error below, even though I did not delete any vertices and made sure the cage is as close as possible to my inner cage, as Roblox suggests.

Any help with any of these would be SO greatly appreciated. Thanks!