I've found that 'coating' the tip of the screwdriver with glue from a glue-stick will help to keep the screws from coming off of a non-magnetized driver.

I just now attempted to edit Step 17 to tell how I dealt with getting one of the brackets loose.

=I was able to easily detach the upper/top-side bracket; but, the bottom one [towards bottom of the iMac] was another story! So after detaching the upper bracket, I loosened the 2 screws holding the 'bottom' bracket to the drive. Then I was able to lift the drive enough to easily detach the bottom bracket from the metal framework.