Allen, I don't have a Simplicity, but if I interpret your pic and comments correctly, that blade hole is much larger than the spindle's mounting. If a bolt and washer hold it on, it will still allow for a lot of shifting around under the load of cutting grass. The wear pattern on the surface of the blade implies that is happening.
In this pic of your pic, is the "slop" you described that gap where the red arrows point? If so there is something wrong here. You might check with a couple blade manufacturer web sites, designate your model, and see what the "correct" blades look like? Most will also have specs listed that will show both the size
and shape of the blade mounting hole. It is also possible that if this is a used machine, that someone used incorrect replacement spindles and blades.