Wing dihedral

Once the aircraft has stopped rolling, provided it is still travelling
straight ahead, the aerodynamic forces will be influenced only by the
airstream passing over the aircraft. This will be identical for both
wings and so no restoring moment will result.

This is incorrect. The upper wing will have a lower AOA, regardless of whether the plane is slipping or not, and therefore will produce less lift. The higher relative lift of the lower wing will stabilize the plane.

Let's consider the "90 degree" dihedral wing mentioned earlier. Let's assume that when the wings are "level" they produce enough lift to fly level at the current airspeed. Let's assume that one wing is horizontal, and one vertical.

The relative wind is coming from ahead, and from slightly below. The vertical component of lift on the upper wing is now directed inwards, and is therefore no longer supporting the plane's weight, so the plane starts to descend, increasing the AOA on the lower wing even further. The AOA on the upper wing remains lower than normal, because the relative wind is hitting it diagonally. Note that due to the relative wind, the dihedral would right the plane even if it weren't descending.