I only discussed KK in my IJ essay (ie no other Coleridge poem).
I think what you're referring to comes under a number of different points;
1. Firstly, Coleridge/Kubla is attempting to construct a manifestation of perfection. By definition, this is a concept which does not exist (at least not on earth), hence the reader is taken on a journey of attempting to understand/cultivate this concept, only to arrive at the conclusion that it can only exist within the realm of the imagination.
2. Charles Lamb (STC's mate) expressed the notion that hearing/reading the poem deports the reader on an imaginative journey, as the imagery, metaphors, ryhming pattern etc stimulate the imaginative capacity. He wrote of hearing STC recite KK "... so enchantingly that it irradiates and brings heaven and Elysian bowers into my parlour..." NB DIRECT QUOTE
3. Whilst it is STC who articulates the words, it is the readers imagination which gives them form within the mind. Eg... five miles meandering with a mazy motion (not sure if that's right... memory faded since HSC!!). The colours, sounds, smells, sights which we attach to the words are products of our own imaginative journey, whereby we create Xanadu in our minds.
4. As KK learns that he cannot constrict nature (ie the fountain etc rises up against his constraint), so too does STC learn he cannot harness the imagination with words. This, in itself, is a journey with which the reader joins STC, in the understanding that the imagination and creative capacity are facilities beyond our control and understanding. In this way, the IJ of KK is parallel to that of STC and the reader.
Hope that helps. I'm hoping I haven't given you a bum steer. I got in the top 20 in the state for 2u, so obviously the markers liked some of the bullshit I spun!!!
Good luck and feel free to PM me with specific questions.
xx