My brother has hostas under older river birches - it is a struggle to keep the beds watered - river birch is a water hog, and has a massive root system. Magnolias have fleshy, brittle roots and don't like to have them disturbed, so I don't think they would work. The Sweet Bay may be different than the M. grandiflora ones, though. I don't know if redbud would do for you, but I find the branch structure of mine interesting in winter. If you found one of the crab-apples that held onto its fruit in winter, that might be interesting. What about a thornless honey locust - its shade is open and light? Whatever you buy, unless you pay out BIG money for BIG trees, you will have a wait for the shade. While some of the hostas will adjust to more sun, you might think about building a pergola over the most vulnerable ones, and covering the top with shade-cloth for the 3-5 years it might take for the trees to grow large enough to shade the 20' wide area. You could also grow vines over the pergola, although they might not give enough coverage. You might like the look well enough to leave it and have only one or 2 trees there!

How thick is the retaining wall? If it's planted only a couple feet behind the wall, it's only a matter of time before the wall cracks and crumbles. I know around here it happens all to often... retaining walls aren't meant to retain a massive root system. Even dwarf trees, given enough time, will knock it down. It all depends on time. If your 80 years old it doesn't really matter so to speek lol... if your 20 years old and plan to live in the house for some time almost any tree will cause damage. What about some tall grasses to block out the neighbor. The type of grasses that get 5-6' tall with pink or white flower spikes then some cool stuff like Juniperus horizontalis 'Mother Lode' that drapes over the wall?

How about a shoe string acacia? About 20 feet tall, 10 foot spread, and fairly rapid growth. Is evergreen. Rather open shade-not real dense. Low water needs and needs little or no pruning and not messy! Sorry I can't recall the scientific name but any southwestern nursery should know what you are talking about.They are from Australia originally. Good luck, Bill

I think that my strategy would be to plant a few small trees rather than one large one, far enough from the fence so as to not get into the wires. If you put garden beds around the trees or more accurately put the trees within garden beds, she won't need to worry about mowing under them. As far as placement, put them where they will provide desired shade, but far enough from the fence and wires so that they won't cause issues. Exactly where that is will depend on how she uses the yard and the particular trees used. Does she want room for a dog to run or to sunbathe or a volleyball net or . . .? Is she planting a tall slim tree such as the columnar sweet gum Liquidambar styraciflua 'Slender Silhouette' or something shorter and broader? The Fox Valley river birch AKA Betula nigra 'Little King' could work for you. IME they drop a fair number of little twigs, but that they are fine enough so that they can be mowed over by the lawn mower. Fastigiate or columnar trees could be taller since they will grow narrower. There are a range of flowering crabs, and some are quite short and have quite small fruits that are readily eaten by the birds; the fruit that does drop I wouldn't consider a mess unless it was falling on pavement since they are small enough to disappear into the garden plants below. She would want to avoid larger fruited trees. You could also look at some of the larger shrubs and limb them up. Large panicled hydrangeas (H. paniculata) are often pruned to tree form, and some are large enough to form small trees. Look also at other large shrubs recommended for your area and research to see if they respond well to tree type pruning. Check out smaller Japanese maples for your area, especially some of the hardier species such as Acer japonica. Another alternative would be a trellis with tall vines such as Lonicera sempervirens, a native honeysuckle, growing on it. Here is a link to the USDA zone map. You can type in your daughter's zip code to get the zone, but generally Chicago is zone 5, either 5a or 5b.