So I installed this shelf. It’s the black wiry thing:
But it was a bit of an ordeal. I managed to snap not one, but *two* screws while bolting it in.
The drill bit on the left was a casualty of trying to repair the mess. One screw shaft was irredeemably stuck in the wood, I couldn’t get it out. The other one I needed to drill around to get it out, it was stuck hard. Then I just plugged the hole I had created with craft sticks and hot glue, then tried again.
The spirit level says I did a fairly reasonable job of it: