I don't have a design background like many of the folks here do, but I have dealt with a problem similar to yours. I've planted Thuja Green Giant here and elsewhere, and per the name, they get big very fast. Unless your neighbors are on a hill overlooking your property, the thuja are way more than you need. I don't know how their roots would be when they are large, but I believe there would be a risk they would over time start to lift your wall. An inch would be more than enough to crack it. I imagine the frost line there is not very deep? How high a privacy screen do you need? If it's just 12-14 feet, there are many shrubs that would work, with essentially no risk to the foundations of any structures. Plain old forsythia would work. Or do you need an evergreen?

The way to get tender roses through cold weather is to cover the tops with mulch or soil. The last time it got below 10 degrees F here a lot of roses died back or died - I would definitely do something to prepare for 2 degrees F if I had roses that were kinds I was concerned about. The temperature ranges for USDA Hardiness Zones are average annual minimum temperatures - individual temperatures being experienced at particular times don't really have anything to do with them. Think of your USDA low temperature spread as your core set, your mid-range, with it being normal for some winters to fall outside of it. The USDA web site tells people to plant stuff rated for the next coldest zone. Part of why this is a good idea is that so many commercial (and sometimes other) sources rate plants hardier than they actually are. It seems much of the time this may be due to a misunderstanding of what the USDA Hardiness Zone temperature indications refer to. This post was edited by bboy on Sun, Jan 5, 14 at 17:58

I leave my astilbe alone until Spring, then I clean up any remaining foliage that's still there as the plants will re-grow from the ground up. Leave the coral bells alone. Mine don't even die back all the time. Just remove dead leaves in the spring. The goldflame spirea you can just leave alone as it probably doesn't even need pruning yet. Of course, I've had rabbits prune mine back in hard winters, and it just keeps thriving. My weigela lives on neglect and does super. Some people cut the hosta back to the ground in the fall, but you don't have to. You can clear any lingering dead leaves in spring cleanup, which is what I do. I also pretty much ignore my daylilies, except I deadhead the tigerlilies when their stems turn brown in August. My other daylilies I don't do a thing to, ever. The monarda comes up from the ground in the spring as well, and I never bother doing anything with it in fall.

Trees planted too near walls, walks, and foundations can eventually shift and break those constructions. I have a low dry-stacked retaining wall and some twenty-odd years ago I planted a red oak about 3 ft away from the uphill side of the wall. Today I have a beautiful tree, but the wall is being shifted by its roots. A north-facing area may be in shade all or most of the day, but if it is open to the sky it is considered to have medium shade. Daylily, fuchsia, wax begonia, impatiens, wood anemone, oak-leaf hydrangea and some rhododendrons should grow well here. The trees you refer to will in twenty-odd years shift your wall. You can mitigate this a little by leaning your wall rocks into the hill. HERE, HERE, and HERE are drawings that show properly stacked retaining walls.