FWIW -Here is my experience. 16 years ago I built a modern day version of what you have with the HUGE exception of my "wishing well" only being 18' down. We needed a water source. I brought in a trackhoe and my contractor dug an 18' deep hole. Then he took a 20' double walled, 4' diameter, polymer culvert and stood up into the hole. The culvert stands two feet out of the ground. It is grated with aluminum, covered, and boxed. There is virtually no way somebody could fall in and almost zero chance of a wall collapse....to your project
1912 construction, WOW very cool, and WOW very old. My first move would be Safety first. I would get a good solid "casing" down that hole. And make sure it protrudes above the ground and is covered. This will help protect against somebody inadvertently falling in and wall collapse. When you start removing material there is no way to know what the rest of the structure will do. With a casing in pace, if the sides do collapse you will still have your well. Look up the story of baby Jessica (1987?)and it will send shivers down your spine.
