I think NXIVM is not meant to be nex-ee-um, but rather nexum, and the I is a dividing line:
nexum noun
nex·um | \ ˈneksəm\
plural -s
Definition of nexum
Roman law
: a formal contract of loan with coin and balance in the presence of five witnesses under which the obligor could be seized and held in bondage for failure to perform
SNCTM also doesn't use vowels
SR/CL also no vowels and uses a dividing line
YHWH is another example
The I in NXIVM just doesn't make sense