There was some discussion over whether the decorative scroll work would be set flat against the stringer. The owner opted to avoid any further cutting into either the treads or the risers. It then leaves a small space open in between them and the stair, creating a shadow. This only meant an adaptation to the returns of the store-bought tread returns. I would mitre the front side and leave the back side alone, butting a small trim piece onto it underneath.
The picture below perhaps best illustrates how this would be accomplished. The warpage out of squareness was compensated for by marking and chiseling off the deviation. The risers were capped off with stained oak cove molding, square cut on top of the tread returns, and mitred at the top to cover a spacer which turned the tread return into a classical profile underneath. It then made a short return and butted into the stringer.
This gives an impression of what it will look like to whomever buys the house when just coming in the front door. I can additionally imagine maybe a side board with some kind of vase and flowers or sculpture on a small pedestal.