Sounds good to me. +4 racial bonus on Move Silently?
"Undissmissible" does not appear to be a real word (nor does "indismissible"). Can anyone think of another name that works for that ability (or is it OK since it is based on a D&Dism)?
CR 5? Although lower HD than a phase spider, they have a wide array of abilities and are far more maneuverable.
Treasure: J,K,L, and M translate to just a handful of coins of each type carried by individuals, but B looks like standard treasure in lair.
Advancement: 5-12 HD (Medium)?