Mac Designer: how to ungroup lines that aren't grouped

[VectorCat]

I made a shape from lines which I expanded, then booleaned to combine. Now I want to delete all but the outermost path. Yet, no matter how I break up the inner paths, they all get selected when I use either arrow. They aren't grouped and "release compound" is greyed out.

Any idea as to how to isolate these paths without clicking on every point?

Thank you!

[Gear maker]

Does a Divide do what you want?

[G13RL]

Do "Reverse colors" before "Add". Then select the shape, "Layer""Geometry", "Separate Curves" and delete the inner shape.