Answer: Your hostas are probably suffering from transplant shock and if you keep the soil uniformly moist they should settle in and produce healthy new foliage.
Oriental lilies provide an easy to grow, colorful addition to any garden and by choosing a combination of early, mid-season, and late-blooming cultivars, you can have lilies in flower from mid June through mid September. I think your lilies have simply finished their blooming period for this year. You can cut off the flower stem but leave the foliage alone. It will die down on its own and then you can remove it from the garden.
Peace Lily (Spathiphylum) may tend to develop yellowing leaves if you over or under water it. Keep it moist, by giving it a good soaking and then letting it dry out on the surface two inches of potting media before watering again. When these plants get too dry or stay soggy wet for too long they often get yellow leaves that soon die. When you first repot a plant is it easy to overwater as there are no roots in the new soil to take up the moisture. The plant does not need too much light, so be careful not to expose it to direct sunlight or even really bright indirect light.
As for planting under your tree, a few annuals that tolerate shade or partial shade are: nasturtium (Tropaeolum), flowering tobacco(Nicotiana), pansy (Viola), lobelia (Lobelia), bachelor button (Centaurea), Pinks (Dianthus), bells of Ireland (Moluccella), forget-me-not (Myosotis), baby blue eyes (Nemophila), perilla (Perilla), fuchsia (Fucshia), and elephant ear (Colocasia).
Best wishes with your landscape.
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