Those screws cover the side parts of the wing that cover the glass attachment. I had to take them out at the bottom because the smaller "wing" that you push on to the close the trunk needs to come off to get the main wing off, and it slides under those side wings. The side wings are held on with a funny circular sort of slotted nut and you only need to take the ones off near the bottom. The main wing nuts (ha ha!) are four larger ones that clearly go through to the wing itself. Take them off and the wing comes right off. It is awkward and a bit heavier than you might think and will tilt if you don't hold it firmly. I would hate to see a pic of it through your back glass.
There are four parts - the main wing held on with four (or perhaps fewer) bolts, the centre smaller metal wing (there is a bit of wiring under there for the wiper motor) and the two side wings. They all come off very easily but the side wings only need to come off at the bottom if all you want to do is remove the main wing. If you want to remove the glass all parts need to be off.
If those screws are spinning it is because the threads on the female end have stripped or the nut has fallen off (can't remember which). I simply replaced them with a small bolt and nut with a washer.
