Thanks for the feedback! I definitely understand emergency evacuations off of rides firsthand and that was one of the first things I thought about when deciding to go underground or not. An elevator and stairwell should be implemented as well as fire suppression at the bottom. Evacuation procedures for a drop tower stuck half way up require a team to repel down from the top to the catch car. In this case, the repel team would be the same but there is no catch car but it is a continuous cable system with the air piston. In the minor likelihood of the cables breaking, it will brake at the bottom and guests would evacuate up the stairs or elevator.
But yes, I'm not sure on the technicalities of digging a 50 foot diameter circle down 120 feet inside an already existing building... the roof would certainly have to come off for the entirety of the construction. Perhaps make it less deep too?
Maybe like 50' or so down assuming the water table would allow for it like @Nicole mentioned would still be thrilling and allow for less safety complications and lower costs.
I think you're correct in that the building may be gutted with the roof (and probably the staff rear entrance) removed to make way for right and crew.