The 2 screws in the rear portion, #81 and 101 - screwed in horizontally, are in very narrow holes, you may not be actually turning them without a slim driver. I have a particularly thin-walled bit driver that I use to get in there, my normal ones do not.
I have seen more than one of these with one or both of those screws broken out by previous attempts, if that happens just remove the screws once you can access them and use a longer screw with a washer in it's place, just don't pierce all the way through to the the freezer wall.

If you are certain you have a grip on the heads apply some force to the plastic cover in the direction it comes off as you turn them, it's possible the holes in the liner have been stripped.
Here's your diagram though-

Fantastic! Although I make my living fixing these things I love it when a customer can do it themselves. That's how I started and gained confidence enough to start charging people for that knowledge.