This would be an interesting place to visit. I can imagine the party arriving just as the 'Court' is about to leave after a few days of shenanigans. The party could dig up all the dirt on the nobles and use it to there advantage.
I did not give the association with the thieves guild a lot of thought. It originally was only a twist on the end but I added a few other links as I refined the piece. The ended up being a 'sneaky' way of getting to an end. In my mind, the increase in tavern business would make more money than the well as local people would come to gossip about the well.
I could say that the guild could press for all the money from the well knowing that the tavern is the money maker only because of the well. But that is not what I had in mind to begin with.
I do like your idea of an alternative reason for the guild wanting the well very much.
I used this in a game with four young boys once and it was the best part of the game. It had nothing to do with the plot but we did spend a lot of time making wishes. Oh, the things boys can come up with. They never investigated the well or anything of the sort. They just spent there money and made wishes and laughed for almost an hour. These games are for having fun after all. Go to Comment
Keeping people out of the well is simple. It is barely big enough for a bucket let alone a person.
There is a wooden tub with water at the bottom that the coins fall into for the 'slash' and they would be very easy to get out. Also since he gets to the wooden tub through a tunnel from his basement no one will see him do it. Go to Comment
This piece is not meant to be a literary masterpiece. I take this as it was meant to be, only seed for thought. I happen to like several of these ideas and I also like the way the are presented. A sentence or two and my mind can take over from there.