Personally, I would just do that kind of a reveal using an animated mask (yes, you can add a mask to a shape layer), or a track matte. Trim paths are only really useful for simple strokes. If you want to reverse the direction of a trim path, then you can simply change which property you animate (start vs end) or use the vertex you want to use as the first vertex then use the menu Layer>Mask >Set First Vertex. You can also add a trim path animator to each path in a shape layer. About five minutes with the User Guide (type trim paths in the Search Help field at the top right corner of AE) will give you details.
This screenshot shows everything modified to create 3 stroked lines apply trim paths to the center line, then prepare to change the direction of the animation from left to right to right to left by changing the first vertex:
After the first vertex has been changed:
