If not, I wouldn’t tell your husband after he did all that digging.

Is there going to be a planting bed there? With plants, the concrete won't be noticeable.

Just put about 2 inches of gravel over the foundation that you to hold then you can plant some flowers in the dirt area

There used to be Boxwood bushes there but we had to pull them up last year. We weren’t going to replace them with anything since the nandinas were there. We initially put gravel over it but it all washed forward into the bushes and exposed the concrete again.

I should also mention that we have 3 kids who like to run back there and play tag and hide and seek so that may have contributed to the rocks not staying in place.

If you had put an edging around that foundation, your rocks would have stayed in place. I would do that now, and then replant that area, so it looks good, and put edging and mulch around them, and let the kids help, so they know how much work it takes, and then tell them, not to use that area for their hide and seek.

Do not put anything there that would prevent rain from flowing away from the house. Find a permeable edging then add pebble gravel.

I don't think that is an "overpour," it appears you have shallow frost depth and that is your footing. It looks like the footings were poured being just dug in to the soil, where the soil formed the sides rather than using formwork. If this was the case, then the grade on the side of house was lowered, and shouldn't be, because then you'd have washout from under the footing.

However, I notice the bottom course of brick looks to be 1/2 covered in concrete, so it's as if this concrete was poured after the house was built.

So, for me the pictures don't tell the whole story, but in any case, I'm not sure why you'd need a retaining wall just to hold back some landscape covering (your decorative rock). The retaining wall certainly won't hold back a foundation, and you should not raise grade up above the bricks without some waterproofing, and there should probably be drainage involved. Not sure if this is headed in the right direction for the sliding rock problem you have.