Namkung, Ju. Phonological Inventories of Tibeto-Burman Languages. 1996. hh:hld:Namkung:Tibeto-Burman
"Some retroflex phonemes also exist in words borrowed from Indo-Aryan languages, but are used irregularly."
Nasality is apparently written but non-contrastive.
Diphthongs can only occur in open syllables, so they're considered to be VC sequences here.