Your 2nd point is easily answered, it has to deal with the crit mechanics of dot skills. If the first tic crits, the entire dot will crit. If the first tic does not crit, the entire dot will not crit. The times you feel like you arent doing any damage is because you are activiting soul harvest on a dot which didnt crit.
For some more clarifcation, Quezacolt will doube your damage as it doubles the rate of the dot. Creeping death makes your dots last 5 mins but Jade Harvester's soul harvest will only consume 30 seconds of it. It's done this way to balance it, or else if you'd consume the entire 5 min dot, you'd one shot everything.
Regarding treasure goblins, be aware they have a brief period where they are invulnerable. After you initially hit them, they wont take any damage for 1-2 seconds immedietely after. Make sure you arent hitting your soul harvest during this time.