Yes — LayerCake is also available for free on kunemann.com. This Superhive listing exists only as a way to support development; buying it here is entirely optional and gratefully appreciated.

Blender's Combined pass is a single baked result you can't take apart in comp. LayerCake reconstructs it from the underlying passes as editable nodes, so you can rebalance diffuse against glossy, mute a light path or regrade emission without re-rendering.

Not if your passes already exist. From a multilayer EXR that contains the passes, LayerCake rebuilds instantly with no re-render. On live Render Layers it can auto-enable any missing passes for you, but Blender only fills those with data on the next render.