I'm not aware of any words beginning with ん, it's usually placed between two other morae. If the following mora begins with a stop, it acts as a nasalization of that stop, similar to English n in "blank" vs. "bland". Otherwise, such as for んあ, it nasalizes the preceding vowel. E.g. the name 闇亞 (あんあ) is pronounced [ãa], whereas あな would be pronounced [ana].

I'm also not sure what you mean by kiyu and kyu, since きゆ and きゅ are clearly orthographically distinct.