The ideas for the vulnerabilities look good to me.
Had some ideas on the special ability. One question, though: the 2e spell is a touch confusing as to whether the caster needs to continue speaking or not. At one point it says he/she does, and later it specifically says that the subjects are still affected even if the caster just up and leaves. It's probably more flavorful to force the mephit to keep talking, so I'll put that in. Here's another attempt, simplifying further.
Harangue (Sp): An ash mephit can harangue good and neutral creatures with a lengthy recitation of its woes once per day (this effect does not work on evil-aligned creatures, because they do not care about the mephit's troubles). The subjects must make a DCX Will save or become engrossed in conversation with the mephit and each other as if
fascinated. On the 7th round, if the mephit is still speaking, each subject must make an additional DCX Will save or fall into a rage, attacking all the other conversationalists (except the mephit) for 4 rounds. This effect lasts until the mephit stops speaking (a standard action each round) or for 10 rounds, whichever is less.
This is a language-dependent, mind-affecting compulsion effect. The save DCs are Charisma based.
Does that look a bit easier?