The full title of the poem by Samuel Taylor Coleridge is "Kubla Khan; or, A Vision in a Dream: A Fragment," named for the nature of its creation. As the story goes, Coleridge was taking an opium-infused medicine and reading a book about the emperor Kublai Khan when he fell asleep. Upon awakening, he attempted to write down as much of the dream as he could remember.
The answer to your first question can help inform how you choose to answer the others. The "underworld" is a common trope in religious, mythological, and cultural thought, and as a concept is almost ubiquitous in every culture. Generally speaking, it's the land of the dead—often located within the earth itself. As such, the underworld can be associated with darkness, fire, and other aspects of the subterranean. It's also common for myths to involve a living hero making a journey into the underworld and returning. The underworld can be seen as a mirror to the land of the living: a balanced, opposite realm when compared to the world above. In some cases, the underworld is actually the source of life.
How do these common characteristics of the underworld find their way into "Kubla Khan"? The second stanza introduces it quite literally, as the reader is taken on a journey into a "deep romantic chasm" near Kubla Khan's palace, described as savage, holy, and enchanted. A great fountain bursts from the chasm, forming the river Alph and ending in a "lifeless ocean." In the chaos, the voices of the dead can be heard "prophesying war." You might also make the connection between the dreamlike, drug-induced state in which the poem was written and the nature of the underworld.
There's a tension between these darker, more violent aspects of the poem and the whimsical, paradisaical descriptions of Xanadu and Kubla Khan's palace. You could interpret this as representative of the dual nature of heaven and hell and the microcosm of light and dark within humanity.
I would suggest exploring these themes further, then relating them to the other questions you've listed. Knowing what you've do about the underworld as a general concept and as explored in the context of the poem, what might Coleridge be saying about the dangers and experiences of venturing too far?