If you remove the cover make sure it isn't connected to the heat sink which is bonded to the GPU. If you do that then you will have to get thermal paste and reeat it.
To get the screw out is going to be a problem. Most likely you will have to drill it out. I don't know of any tools except easy outs but they are for bigger screws. Can't you just use compressed air and blow it out.
Good luck to you.