Greetings! I playtested your quest using 3 characters and both vanilla and OpenMW engine. The quest is quite entertaining. I tried hard to search for bugs and I'd like to mention the following things (I'm not a reviewer):

1. It's not very clear that the questgiver is just a courier who wants to delegate her task. Especially for the PC who doesn't yet know the Dondril tavern keeper by name. She could use some more introduction about how she got to deliver this mazte. Ideally, it would be better if Bols himself gives this quest.
2. In the questgiver's response to your refusal there is a missing period in the end of the phrase.
3. Directions to the mine have an unclear point with the tomb (both the journal and the dialogue directions): there will be (a) tomb on the right, the next time the road forks the mine will be on my right. -> should probably mention, that the PC should not take the turn to the tomb. But maybe it's only me who misunderstands this.
4. Ilvy Mine foreman topic gives confusing directions: just inside the entrance to the mine makes me think he stands near the load door (interior's entrance). But I understand that it's hard to give directions in the mane since it's quite uniform in its design. Maybe the miners could simply give the PC the name of their foreman and the player would simply find him by looking for Enar Salavel.
5. When you meet the foreman, he makes you solve his problem with a missing miner. But it doesn't sound natural -- a courier should not be pressed into solving such problems. Instead, there could be an option to refuse to help him. In which he simply gives a smaller reward for the delivery.
6. On OpenMW when you give Goval a health potion, he speaks as if he was a receiver of the mazte delivery. I believe, it's an artefact of the previous iteration of the quest.
7. Goval's initial greeting sounds wordy. Consider rephrasing the second sentence or dividing it into smaller ones. Aim to keep sentences less than 20 words long.
8. On vanilla engine Goval doesn't recognize a Standard Health Potion in my inventory (ignore the bottom line, it's from a mod) when I press the spare potion topic. Also, having him accept only the Standard Health Potions is a bit contrived (I didn't check the others though). I suggest adding him responses for other types of healing potions as well: bargain, cheap, quality, and exclusive. Feature creep suggestion: make him praise the taste of the expensive variants and complain about the cheap ones. An option of healing by a spell works great, and I consider borrowing this cool script for my personal mod :D
9. When I find Goval and inform the foreman without healing him, the quest ends (he takes the mazte from me). But when I go back to Goval, he acts as if I've helped him.
10. Goval Thadus topic remains outdated after I heal Goval and inform the foreman. Add him a new comment, like: I am impressed, that you went so far as to not only find Goval but also help him. Good job.
11. Nathala Randaro has no new comments about the job. While it's totally ok, there are two ways how to make it even better: 1. (The easiest) Disable her when the quest is finished. Implying that she's a freelance courier who moved on with her work. 2. Add her new comments under the quest topics. Like: So, you're done with the delivery? Good. I hope they payed you well. And: The foreman's name is Enar Salavel? I should probably note that for future work.

Feature creep suggestion: Help Goval to finish his initial expedition -- if the player kills the kwama forager behind the waterfall, Goval can have a new greeting or topic, saying: You cleared that alcove? Very brave of you, thanks. We can work with the glass deposits there now. Accept this [minor trinket/dagger/jewelry]. I found it underwater while exploring these passages.

General feedback: I like how this quest adds content to both Dondril and Ilvi Mine. These locations lacked misc adventures before. And you incorporated the layout of the Ilvi Mine very well in your story (with the hidden path behind the waterfall and so on). I wish you well in your future creations, and I hope you'll work with TR.

P.S. I'm not a native speaker, so disregard my dialogue suggestions when there's wording or grammar issues.