Your spondylolisthesis is due to your fall, as I said previously. You had no abnormal findings on your original MRI except for the disc herniation. You don't mention that you have arthritis in any other areas of your body which, if your arthritic changes are due to the normal aging process would likely show up as osteoarthritis elsewhere in your body e.g. in your hips, cervical spine, shoulders, etc. I don't believe it's a coincidence that you would have arthritic changes due to normal aging specifically localized to the area of your original injury unless the injury caused the degenerative changes. Spondylolisthesis is a side effect of a ruptured disc. It makes no difference whether you have 4, 5, or 6 lumbar vertebrae, you can have a ruptured disc if you fall from a rooftop or from a ladder. Of course, the further you fall, then the more likely you are you will rupture a disc. When a disc is ruptured then it affects the stability of the vertebra above it (graham cracker/ice cream example).
Your slipped disc could not be due to your hysterectomy.The vertebrae are not held-up by the uterus. The vertebral column is held in alignment by ligaments which have nothing to do with the uterus. Removal of the uterus does not cause spondylolisthesis (slipped discs). On the other hand, many women who have hysterectomies complain of back pain: In one study 51.9% of women who had their uterus removed (with or without removal of their ovaries) complained of back pain as an adverse outcome following surgery http://www.hersfoundation.com/effects.html#chart4. Here are links to diagrams showing the relationship of the sacral spine to the female reproductive organs:
Spondylolisthesis can occur weeks after the initial injury or years after the initial injury depending upon several variables e.g. the stability of your spine prior to the accident, your activity level following the accident, your posture, your weight, the extent of the original deformity, the condition of your vertebrae before the accident, the progression of the disc rupture, the degree of tilt of the vertebrae, etc., etc.
Fusion is the usual treatment to prevent your spondylolisthesis from progressing and to eliminate any abnormal motion as a result of your spondylolisthesis.