Please take a look at the attached file for a possible solution. I have moved the spline to the ground as it is easier, I hope that isn't a problem.
The style uses 3 generators, one for the coping, one for the sloped wall and another for the upright wall. The straight wall and the coping are pretty straightforward, while the sloped wall is only slightly more complex as it uses a trigonometric expression to maintain the position in relation to the spline and cap stones as you increase or decrease the number of courses. I've also added a couple of expressions to calculate the Z Offset of the cap stones, and the position of the back wall automatically.
To follow the contours of the surface I'm afraid I've had to use our Glue
utility to conform the spline to the surface. Unfortunately RailClone's surface feature doesn't work with an A2S generator with the X Rotate, as is used in this case (I'll look into this).
The attached file has some notes to explain how it works but please let me know if you have any further questions.