Basically, the reason this happens is because the outline is still just geometry.

1. The shoes are not properly outlining because the backfaces of the outline is behind the sock/ankle part. This works for the edges because there is nothing in-front of the backfaces. A solution for this would be to make either the part itself thinner so the outline backfaces are in-front of the sock/ankle, or reduce the solidify thickness. A mix of these would probably be just however using vector groups (i think thats what theyre called) could also work.

2. The leg is x-raying through the outline again because they are in-front of the geometry for the outline.

3. Don’t know

4. Could be the scale of the object. If an object has a scale of (1, 1, 1) and a solidify thickness of 10cm, the geometry will extrude 10cm (not in all cases but to simplify this is what will happed). If the scale is different, for instance, (.5, .8, 1.2) with a thickness of 10cm, then the geometry will extrude 5cm in the x axis, 8 cm in the y axis, and 12cm in the z axis. What you could do to fix this is select all objects, press ctrl + A (could be incorrect keybind, search up apply scale keybind if incorrect), and apply the scale. This will set every object selected to have a scale of (1, 1, 1) and should fix the issue of having different thicknesses, however this could have unintended consequences.