Thanks. I got stuck at this point. I'm doing a complete case disassembly, and re-using it to house an electronic project. This case is built like a tank. There's military electronic gear I maintained that was easier to disassemble.
Now that you have the shelf out, it's fairly simply to tear the case down to the component level. Once all the theaded fasteners are removed from the perimeter of both sides, the far side of the case can be removed. All that will be left is a bunch of rivets you can drill out.
Now it's time to re-work the front and back for controls, indicators, and connectors.
I think I'll name the finished project 'Wasamac' in honor of its parentage
Just remove the circlip at the top with a small screwdriver by prying against the pin. Put your finger over it so it doesn't fly off into the wilderness. I use needle nose pliers against the pin and back of the circlip to re-install them. They snap right back into place without any problems.
If you are going to re-work the case for another project, remove the welded in parts by twisting them gently with a pair of pliers along their lengths. You may need to work along the welds several times before they pop out.
I'll be using washers as spacers for the side panel locking bar rather than re-using the shelf.
It looks like you have removed the circlip. The screwdriver went between the side of the pin, and inside of the clip. There are two cutouts in the clip for this. The pin should just push through, but the shoulders may have caught on the latch or delrin washer. Try holding it between your thumb and forefinger, then wiggle and push it out. You may have to re-align the washer as well.
You can see the end of the pin where the C clip was, this pushes away from you (into the case). So using the flat blade screwdriver, go to the other end of the pin (which is shape like a T) and lever the pin towards the inside of the case. There are two (nylon?) washers / bushes, make sure you don't loose them. One of them may get trapped inside the C-clip groove, so them to make sure the pin is centred as it passes through.
Many thanks to the OP, FINALLY stripped my G5. Now contemplating remove ALL those screws to get access to the back of the motherboard posts.