Why are English words ending with -ing often transcribed with ング? For example, timing = タイミング, morning = モーニング, and diving = ダイビング. My guess is that グ is formally pronounced with 鼻濁音｛びだくおん｝ , i.e. pronounced like /ŋu/. Therefore, is this the case?
You're on the right track, but a little off.
When a language borrows a word from another language, it has two choices: drop the sounds that don't exist in their language, or add sounds to preserve the original pronunciation.
Japanese is a language that tends to try to preserve the pronunciation.
So, グ will not be pronounced as /ŋu/. However, /n/ becomes /ŋ/ before /g/. But since /g/ in isolation isn't possible in Japanese, they employ /gu/ (グ), in order to force the /ŋ/ pronunciation of ん.
So the phonemic /n.gu/ is rendered phonetically as /ŋ.gu/.
This sound cluster also exists outside of English borrowings: