As low as the Disney system will be, is there a provision to slack the drive wheels and just lower the cabins to the ground?
No this is not possible. One side of the lift will have a tension system, the bullwheel will slide back and forth on a track with a hydraulic ram, in the old days this was done with huge cement blocks. This is done just to adjust for weight on the line and temperature etc. and is not used in any way as an evacuation device.