The heads of the bolts, being moulded into the plastic, don't have much to stop them from turning compared to the torque the rust can apply from the nut. Even if you could cut the nuts off somehow, you'd still have the problem of nothing holding the head when you try to put them back together. When I had those steps years ago I damaged one of them, and the bolt heads were ripped out of the steps. The only way to secure the step was to drill the holes through to the top and put coach bolts through the step. Best just cleaning and painting as they are.

 

No guarantees but try soaking them with PB Blaster for several days. It's pretty amazing stuff if left to sit. The question would be if the plastic will hold when you try to unscrew. For tough nut removal before the plastic gave up it's grasp the quick zip of an impact gun might do it after the soaking. I think I would agree to clean them and paint them without trying to take them apart. You can't see the bottom anyway. If you can your up side down:surprise: