The spine-canvas runtime does not support meshes. I can not load any of the links you provided, but I assume that you are using meshes. You will have to use the spine-webgl runtimes, or the spine-widget for your skeleton to fully render.
As for the iPad 2, it does support WebGL in Safari from what I see in the specs. However, it will not be able to render with a 4kx2k texture due to hardware limitations. Save atlas size limits for mobile are 1024x1024 for older devices and 2048x2048 for newer devices.
The problems you see in Chrome when shrinking your atlas down are likely due to you having enabled white space stripping. Note that white space stripping does not work for skeletons containing meshes. If you disable it, your skeleton plus the shrunken atlas should render fine everywhere.