It's been so long since I removed or replaced a skeleton that I don't remember. Do the plugs hold the skeleton to the tunnel? If they don't, you could remove the skeleton as a whole unit then access the plugs from the backside. Maybe the tips could be squeezed with a pair of needlenose pliers to allow them to release.
I did a google search and found where one skeleton had been disassembled. The tip of one of the plugs was laying on the work surface. It appeared they had taken a pair of side cutters and snipped off the tip, likely replacing it with a new Christmas tree button.