I was finally able to get the impeller off, but only after drilling several holes near the hole where the screw goes in on top. Even after breaking off the top portion (I had replacement parts in hand), it took a bit of effort to finally get the impeller to come loose from the shaft it was wedged onto.

I have seen several videos that claim to show how easy it is to get the impeller off the shaft, but in my case it was very difficult to remove. If I ever have to remove it again, I will probably have to make myself some kind of special tool to go underneath and rock it back and forth till it comes loose. A screwdriver will eventually damage it after all kinds of rocking from the sides. Kenmore, whirlpool, and other manufacturers need to do better than this and provide a tool that can make removal much easier.

Once I got the impeller loose, I was able to get to and clean out the cutter blade area. I found a jagged piece of metal, a circular piece of metal, and a small measuring spoon. Noisy dishwasher problem solved. I put it all back together after cleaning things up a bit, and it's working well now.