I’m a fan of using a loading method that involves sliding the boat up onto the roof, so that the only high lifting you do involves a small fraction of the boat’s weight after it has been pushed partway up there. That may not be a good option with J-racks, but it does work if you have a helper, so that may be something to keep in mind when you do have help, because even with two-person loading, the avoidance of lifting the full weight of the boat up high is a good thing.
As to the use of step stools, I can see their usefulness for installing tie-downs, but as an aid in lifting the boat higher it seems like a bad accident waiting to happen. One simple method that would be much more stable and safer would be to set four 5-gallon pails on the ground supporting a square piece of thin plywood (don’t use thicker plywood than you have to, just to keep things light). This would make a nice platform to stand on and it wouldn’t take up much space in your car. The square of plywood can sit flat on the floor in the back end of a hatchback, but if you don’t have a hatchback, cut the plywood in half for easier storage. Also, four buckets nested together will hardly take up any more space than the smallest sized stepstool.