1) you are correct; you would move the strap and buckles, as mentioned.
However, you could also add another strap, at the smaller circumference spot, on the inner apron, and add two buckles in the desired spots, for the outer apron straps ( some kilt hire places do this, apparently).
2) yes, move ( or install new) the same amount.
3) you should have no problem doing this by yourself.
4) if moving strap and buckles, using heavy duty thread, sew through the holes in the leather, and through all layers of your kilt fabric.
5) I don’t think PV will shrink in a dryer.