When you made the kilt the first time was the Hem in the right place? Was the Bottom of the Fell in the right place?

If so then all you need to do, to lower the waist of the kilt, is to chop off the top. You really don't need to un-stitch the entire Fell if the tapering fit the shape of the wearer.

You will need to change the position of the strap hole as you note. You will also need to re-position the Stabilizer.

And just a small detail that you may not find in "The Art of Kiltmaking". Before you put the waist banding back on, have the lady put the kilt on. See if the fit is good. You may find that it will be helpful to cut the front of the apron down just a bit if there appears to be a bit of pucker in the apron.