Yes adjusting the straps whould probably be your best bet to size your kilt correctly and yes I suggest getting someone to do it as it involves sewing through rather thick leather and several layers of wool. belt loop would be easier to add if you have a swatch of matching tartan. Kilt belt loops are for sporrans and should never be used for a belt as overtime those little loops supporting all that weight, and those WPG kilts are heavy, will allow the tartan to sag exceprt at those loops. From the back your kilt will look off kilter with no repair.

The third strap is superfluous and leaving it off might help eliminate some unwanted movement.

Brace yourself, as our lovely and vivacious Anne the Pleater said use suspenders. A kilt will move throughout the day no matter bespoke or off the rack. A belt with or without loops will lessen this but not eliminate. Now real men (like McMurdo and myself) use pnuematic nail guns to keep our kilts in place!