Loose CPU heatsink on MBPr 15" (pictures inside)

Well I've seen some failures but this is a new one for me!

So you have a few options here, take the system in to an Apple store to see what they say. It should be covered under warranty as it's clearly a manufauring defect. Will they cover it, I don't know. But I would try that first.

As to refitting the stud back onto the logic board, it will take some effort as you'll need to remove the logic board place the board on a block so only the area under the stud is supported as you don't want to damage any of the small components on the backside of the PCB.

You'll need some Cyanoacrylate glue (Super Glue) carefully wet the logic board wall and press the stud back in. I would recommend using a stud press and slowly press it back in.

Speed is your enemy here as pressing too fast will cause the laminates to spit apart.

Good Luck!