Excellent question, though you are only allowed to ask one question at any one time. Before starting my answer, I have included some links below that you may find helpful in answering your question. To understand the concept of nature in the poetry of Wordsworth and Coleridge you need to understand that Romanticism was a kind of revolt to Classicism - the literary movement that came before Romanticism. Romanticism therefore moved away from focussing on reason and man's ability to work out situations and focussed more on the restorative power of nature and how it provides balm to us as mankind. Key to the poetry of Wordsworth is the location of the Lake Distict in England, UK, a place of great natural beauty, where Wordsworth in particular spent lots of time. You may want to read poems such as "Tintern Abbey" and consider what it says about nature. These pointers combined with the links below should help you towards an answer.
To add to the answer, 'The Return to Nature' slogan in the Romantics works both spiritually in terms of a Spinozian pantheism as in the idea of 'natura naturata' being a divine creation through the causal act of 'natura naturans' as well as psychologically and humanistically in terms of the human nature. It is both without and within in two differentsenses. The cycle of seasons in nature seems to symbolize the Christian myth of the fall and the Resurrection. Nature is in time. There fruition and death are two sides of the same coin and beauty and the marvellous sensuality go hand in hand.
The idea of the evil in nature is starkly present in the poetry of Blake, especially in poems of The Songs of Experience like The Poison Tree, The Darkening Green and so on. In Coleridge, the mystical and the supernatural relate to an implication of an evil power in Rime of the Ancient Mariner, Christabel and Kubla Khan. In Wordswoth, there is more faith than doubt, but there are certain passages in Prelude where the darker undersides of imagination, especially its failure and the tyranny of nature are brought out.