How about planting at the top edge of the retaining wall something that will cascade down the front to conceal it.

1. That way, you needn't worry about dog waste harming your plants.

2. It appears there is very little space in which to plant.

3. Although evergreen or woody plants would take a while to cover the wall, there are annuals, such a sweet potato vine for example, that would cover it until the long-term plants grow large enough to do so.

There are things you can spray to deter animals; check your local nurseries and possibly even in the "big-box" home improvement stores.

I think river rocks would look nice where the narrow strip of grass is. I think any planting there would be in the path of human and dog traffic. I second agardenstateofmind's suggestion to plant at the top and have cascading vines. You could even add some archetectural pop by using planters on the ledge.