How do I fix the arm weights going through the spine

You can try manually changing the weights like this, but it would be faster to just change the avatar, run the simulation with hands down and re-do the rest

Also, you should delete all parts of the t-shirt hidden by the shirt. It does nothing but takes up the uv space and increases polycount

The mesh should be vertex painted with 00FF00. That yellow is, first, not used any more (the developers changed it to 3FFF00), second, was meant for skirt-like bottoms and area between breast ( is still meant for this, just a different color)

Never use edge split modifier. It just chops up the mesh all types of ways that will affect how it is shaded. S4S studio will split the mesh along its uv island borders when you import it in the package.

uv_1 is messed up. Use Nearest face interpolated instead of the default option and transfer the uvs from a mesh that does not have doubles removed along the seam that is the uv border in the uv_1