If the buckle is only attached to the one layer of fabric - I'd advise that you sew around it again, through the strap, and the entire apron, that is both layers of fabric and the liner. Otherwise there could be a lot of strain on that single layer, more than enough to distort it.

I would just ensure all the layers were exactly aligned, and then stitch along the edge of the strap. I don't know if there is a name for the stitch, which is back stitch but always inserting the needle at an angle so that on the outside you get tiny stitches across a couple of threads at the most. The lining takes the strain, the stitching just makes everything connected.

The under apron will be hidden in wear, but kilts being works of art you wouldn't want to spoil the look.

A standard kilt is normally able to support itself without a belt, hence all the canvas and lining etc. The buckles and straps need to be firmly fixed to those reinforcements so as to ensure the kilt remains where it should be and not start to give way to gravity.

Anne the Pleater