After having given up on getting the case open, I decided to try again. I succeeded by having 2 tools, 1 to keep the bottom open while I used the other to pry open the sides. After that, replacing the hard drive was easy. I did not need to fully separate it by unseating the headphone ribbon jack.
I'm having a lot of difficulty with Step 2. The tool goes in nicely but I can't seem to find the right tool movement that releases the first retainer clip on the bottom. I never get the degree of separation of the back case shown in the photo. When I remove the tool, it just closes back up. Any help is appreciated.