I think the problem is more than aesthetic. The soil needs something to increase organic content and to slow down water runoff when it rains or your brother is going to have an erosion problem. I hope the footings of that viewing platform hit bedrock. This link might be helpful: http://www.landscape-design-advice.com/landscaping-steep-slopes.html I don't necessarily agree with everything they have written, but it is not a bad start. From what I have read and know from experience, fast growing groundcover is the best way to begin if you can't terrace the slope. Don't think ornamental to begin with. Just cover the slope with something green and low. Even weeds will do the trick. Then plant taller, drought-tolerant and NATIVE plants from there.

no i wouldnt mow it. i would plant a mix of native grasses and wildflowers. Generally its reccommend to use three or four species of grasses and about 10 to 12 species of wildflowers. they dont need mowing but can be weedwacked or burnt down once a year. we also use a mix of clovers and fescues on certain slopes as well...we reccommend that only be mowed, weedwacked or burnt once a year. the longer the height of the grasses the better the root system will be. this picture is stolen from metaxa another garden webber, but it represents the root system of fescue when left to grow to various heights. its apparent from the photo that the taller grasses have thicker deeper roots and to me its intuitive that if water is running down the slope the taller grasses with bigger roots will hold soil in place better than the shorter grass with little roots...

When trimming, keep in mind that this is a matched set of “bookends,” so they should be exact, mirror image duplicates of each other as much as possible. I would think of the trimming project as a challenge … as if you were involved in a manufacturing process making a machine project on a lathe or something along those lines. It would not be at all detrimental to use a tape measure to double check dimensions for matching throughout the process. I would rough out the shapes of both before fine tuning either. The place to begin is standing back well away from them trying to envision what they are to become. If the shrub needs to be moved a little left, right or forward, this is the time to notice it and account for the adjustments you’ll need to make in the cutting. It really barely matters where the plant roots are located. If you the shrub to be 4” to the right, “move” it there with the cutting process. It’s important to envision the finished product before cutting because there is a substantial part of them that won’t get any cutting at all. If you just charged up on them and started hacking away, there’s a good chance you’d cut some wrong parts off. Since the pair of shrubs are matched, the first cuts to make should determine the height. (To match the pair, follow a line on the siding of the house as a guide. Don’t measure from the ground.) Keep in mind that you do NOT want to cut at the place where you envision the limits of the finished product to be (the red line in picture.) There are three reasons for this. As soon as you finish cutting the plant it’s going to grow and if you cut it at the finished height, in a short time it will grow too large. The second reason is because, even though you tried hard to make it a perfect shape, it’s not going to end up as perfect as you’d hoped for. Later, when you need to trim and try to perfect it again, you will want to be cutting only in the newer, softer, easier-to-cut foliage, rather than in the hard, sticky, woody portion. If you don’t cut a little smaller in the first place, you’ll probably end up where the next cutting has to dip somewhere into the woody portion of the plant again. This is a royal pain. If you make the plant a little smaller to begin with, it gives you a “cushion” against having this happen. In general, I find that people have a strong resistance to cutting the plant smaller (the blue line in the picture.) But do yourself a favor and get over this. If you mistakenly cut this plant to a stub, in two years you’d barely remember your error because it would be pretty large again. In cutting it just 6” less than you want it to be, in two months you’ll barely remember or notice that you did that. The third reason you want to cut the plant a little smaller than the “finished product size” is because the more foliage you remove from the top portion of the plant, the greater it is that you expose the bottom portion (the part that is undercut and desperately needs to grow) to more light and an improved chance of quicker growth. Between the yellow line (which represents the ground plane) in the picture and the shrub, there is nothing at all to cut … until you get higher up to the blue line. At this cavity at the lower portion of the shrubs, all the foliage will be left untouched so it can continue to grow as quickly as possible. The goal is to keep the upper portions of the shrub trimmed so that it shades the lower portions the least as possible. Even if you want the shrubs to be larger than what I'm showing, for now you should trim them as I'm showing because you're in a corrective phase trying to regain the lost lower foliage. In your second-to-last picture, it shows how a subordinate, adjacent shrub has encroached on the Yew, which is clearly the more important shrub. The lesser shrub should be trimmed such that this doesn't happen.

VaLady, It's still is a good idea to have that soil mix your landscape guy added tested for nutrients and pH. Here in Henrico Co the local libraries offer soil sample collection test kits to use where you send in a sample to Va Tech for analysis for a minimum price (there are discount vouches available on line in the instructions). Personally I've never cared for "mulch" (as in ground up bark and wood chips) to use in the veggie garden. That type of woody product may be fine for establish perennials like azaleas, rose bushes, etc but it's best (for me anyways) to use real homemade compost as a mulch for growing veggies. Even then adding a balanced fertilizer is required for good growth and production. BTW, I've tried Brussels sprouts with meager results, they just don't seem to do as well as other crops like broccoli and cabbages here. Cauliflower can do well but it can also be a challenge at times. Even broccoli can "button up" when growing conditions aren't to it's liking. Hope some of this info helps, Happy Harvest!