A certain Barovia quest

Since this involves quest info, feel free to edit anything out when responding. I have had some tough quests in my day, but I have been trying to solve this quest with 3 different thief characters, and after maybe 15-20 hours of searching, I am stumped. I have always been above level 20 when trying to do the quest, so I am also wondering if the end of the quest is fixed to a certain level range. I don't want to spoil the quest if there is a solution... mainly I would just like for somebody to attempt it and tell me if either a) it is broken or is missing something, or b) if it stops working after a certain level. If the quest cannot be completed above a certain level range, it would be great if the people that trigger the quest didn't bother to offer it anymore. Thanks.