“toward the hull surfaces” refers to the side of the centerplane which the side and bottom have been created on.
The hull surface is trimmed off-center so there will be a gap in the middle after the side and bottom are mirrored. This gap is filled with the keel surface. See p106 of the tutorial in the Users Guide.
“3. Why trimming the two surfaces results in warped panels.” - Can you elaborate on your question? Are asking why the trimmed panels don’t coincide with the stem curve? It appears that you are using the “developable” option in the Loft command which is appropriate for a boat which would be built from plywood or sheet metal. The shape of the lofted surfaces at the ends depends in part on the shape of the curve extensions. Note the direction of the isocurves in the image you posted. The shapes of the ends of the lofted surface can be changed by changing the shape of the curve extensions.