Hi there! Nice quest idea :) I'm no reviewer, but I did a thorough playtest of your quest (version 2), and found some worthwhile things to report. Don't be afraid by the wall of text, it's mostly typos and additional commas, for better flow. There are a couple problems in the quest's progression, however, which I've listed further down. Here's what I have:

NPC dialogue:

• Dara Thiralas, topic Kephat Rangidil (upon first talking to her about him):
  • typo: "...a skooma addiciton." --> "...a skooma addiction."
  • typo: "...for 'Religious Contemplation,'...": the upper-case can be dropped, here, and the comma is misplaced --> "...for 'religious contemplation',..."
  • flow: "...house arrest for him to sober up." --> "...house arrest, to give him a chance to sober up."
  • flow: "...his family's slave Dro'Jurrzo, but recently Dro'Jurrzo hasn't been able to reach him" --> "his family's slave, Dro'Jurrzo. As of late, however, Dro'Juzzro hasn't been able to reach Kephat."
  • typo: "...  Would you please help me...": there's a double empty space at the start of that sentence.
• Dara Thiralas, topic Kephat Rangidil (upon refusing to help her):
  • flow: "Oh... I see..." --> "Oh, I see..."
  • flow: "Well, I'm sure he's just busy with..." --> "Well, I hope he's just busy with..."
• Dara Thiralas, topic Kephat Rangidil (before having checked on Kephat):
  • typo: "...as soon as you find somethng." --> "...as soon as you find something."
• Dara Thiralas, topic Kephat Rangidil (after having found Kephat's body):
  • typo: there's a missing dot at the end of the player's answer "...as soon as I have something.."
  • typo: "...1 year Skooma..." --> "...1-year Skooma..."
  • flow: "...but I guess you can have it since..." --> "but I guess you can have it, since..."
• Dara Thiralas, greeting (when returning to her after a while):
  • typo: there's an unnecessary ` symbol at the end of Dara's greeting.
• Dara Thiralas, topic Kephat Rangidil (when returning to her after a while):
  • flow: "getting clean too" --> "getting clean, too!"
  • flow: "time to process it I think." --> "time to come to terms with it, I think."
• Dro'Jurrzo, topic Kephat Rangidil (before discovering Kephat's body):
  • typo: "...hush hush..." --> "...hush-hush..."
  • typo: "...khajiit." --> "...Khajiit."
• Dro'Jurrzo, topic go free:
  • typo: "...Khajiiit..." --> "...Khajiit..."
• Dro'Jurrzo, topic Kephat Rangidil (after Dro'Jurrzo confessed):
  • flow: "...no ill wil but..." --> "...no ill will, but..."
• Gols Rangidil, topic Kephat Rangidil (after having found the body):
  • typo: "...Skooma overdose?" --> "...skooma overdose?"
• Gols Rangidil, topic Kephat Rangidil (upon confronting him):
  • typo/style: "...thrice damned cat." --> "...thrice-cursed cat!" (?)
  • style: "...name and if he came home he would just screw something else up." --> "...name. Had he come home, I would have had to live with the shame of his actions again."
  • typo: "...his year long..." --> "...his year-long..."
  • flow: "...mistake I knew I had..." --> "...mistake, I knew I had..."
  • typo: "...that he die..." --> "...that he died..."
  • flow: "...uneventfully than to be allowed..." --> "...uneventfully, rathen than be allowed..."
  • flow (player's answer): "Kephat was an embarassment and your family is better to be rid of him." --> "I agree, Kephat was an embarassment to your name, and your family is probably better off now that he's dead."
  • typo (player's answer): "...Skooma Overdose..." --> "...skooma overdose..."
• Dara Thiralas, topic Kephat Rangidil (after having confronted Gols):
  • flow (player's answer): "...in his house knowing..." --> "...in his house, knowing..."
  • typo: "Oh... Oh my...This is...": missing space before "This".

Journal Entries:

• Upon first talking to Dara:
  • there shouldn't be a Journal Entry for just talking to Dara, in my opinion, since the quest has't really started at this point. Especially not one which states "...She asked for my helping checking...." (typo), since she hasn't actually asked us to help yet. The first entry should only appear once the player selects the topic Kephat Rangidil for the first time.
• Upon discovering Kephat's body:
  • flow: "Kephat Rangidil's corpse." --> "Kephat Rangidil's body." (?)
  • the second sentence is a bit too on-the-nose: I'd rephrase into something more subtle, maybe by just mentioning the pipe, and leaving it at that.
• Upon finding the chest with the suspicious skooma:
  • typo: "...Skooma Overdose..." --> "...skooma overdose..."
  • flow: "...before speaking to Dara." --> "before reporting to Dara."
• Upon reporting Kephat's death to Dara:
  • typo: "...death, apparently by Skooma Overdose." --> "death, apparently by skooma overdose."
  • flow: "...upset but thanked me..." --> "...upset, but thanked me nonetheless..."
  • typo: "...Skooma Eater's..." --> "...Skooma Eaters..."
• Upon getting Dro'Jurrzo to talk:
  • flow: "...for his freedom Dro'Jurrzo..." --> "for his freedom, Dro'Jurrzo..."
  • flow: "...the poisoned skooma and forged note..." --> "...poisoned skooma and a forged note...": I would drop the "the", since it is possible that the player hadn't realized the skooma was poisoned before talking to Dro'Jurrzo.
• Upon agreeing to Gols not to tell the truth to Dara:
  • typo: "I have agre..." --> "I have agreed..."
  • typo: "Gols's..." --> "Gols'..."

Items:

• Unfinished Note:
  • typo: "...and scribbled out words." --> "and scribbled-out words"
  • typo: "It seems as through..." --> "It seems as though..."
  • typo: "You're words..." --> "Your words..."
• S.E.A 1 Year Coin --> S.E.A. 1-Year Coin
• Letter from Dara:
  • flow/typo: "...of Skooma and..." --> "...of skooma, and..."
  • flow/typo: "...Family." --> "...family and ancestors."
  • flow: "I hope that you are..." --> "I hope you are..."

Quest progression:

• Dro'Jurrzo gets the topic Kephat Rangidil, with the line "Yes, this one would pass..." as soon as the player greets Dara, even if the player doesn't actually ask Dara about Kephat Rangidil. This shouldn't be.
• Discovering the chest with the suspicious skooma after having made Dro'Jurrzo confess doesn't trigger a Journal Entry, and makes it impossible to properly confront Gols afterwards (he will just react by saying he isn't surprised Kephat died).
• Maybe there should be a short Journal Entry for confronting Gols again after having told Dara the truth, even though the player promised not to?

Overall remarks:

• The current (v2) ESP has TR_Mainland.esm listed as a dependency. Someone correct me if I'm wrong, but this shouldn't be the case (should be TR_Mainland.ESP, ie the WIP Mainland, instead, shouldn't it?)
• There's some lack of consistency as to the type of place Kephat as been sent to: is he under house arrest in his own house, or in a special refuge, as suggested by the opening dialogue?
• The quests references poisoned skooma, yet it's moon sugar that's lying near Kephat's body. Edit: maybe replace (some of) the moon sugar around the body with Empty Vials?
• I'm not sure "overdose" is even a word that should be used in the world of Morrowind... but we'd need a lore buff's opinion on that. The same thing goes for the Skooma Eaters Anonymous, which might be too blatant a reference to the real world.
• Skooma-Eater should probably be hyphenated, to keep with the style used in vanilla Morrowind, in the book Confessions of a Skooma-Eater, for instance.

That's it! I'd advise you to put up an updated version for review, so as to save the actual reviewer substantial time ;)