/ë/ is inserted as a transition sound between /n/ and /w/.
/ë/ of the inverse suffix is rounded to /ù/ in anticipation of /w/.
The third person suffix /w/ is vocalized as /u/ between consonants.
Note: a transitional /wë/ sequence is inserted after the resulting /u/, presumably by analogy with forms in which /ë/ is inserted between /w/ and /n/.