With heavy clay areas you probably will have to redo completely - remove the plants temporarily and amend with the compost. My friend says she has tulips growing on her trees. The blooms are yellow and orange and produce quite a show. It isn't a cool or warm season grass. The thing it does not like is humidity.
Slugs are everywhere.
Slugs really do damage plants, especially the leaves. Hand pick the larger ones loose in the evening. The beer bait works. You may have to switch bait to keep them from getting immune. Then get the list of trees from your extension agent or nursery that will grow in your area.