If the dealer has measured them, even though most of the corrosion has now gone, they may be too thin and need replacing.
Given rotors are cheaper than the consequences of brakes failing, I would put the news ones on. And have just done so on all 4 corners of my car.
The function of the screws is to locate the rotor in the correct position while building up the complete wheel assembly. The rotor is actually clamped in place between the wheel and the hub, but the wheel bolts don't position it accurately.
If you leave the screws out, you might end up with the rotor off-center and unbalancing the wheel, causing vibration.
Those screws only hold the rotor in place to match the holes for the bolts holding the wheels, and/or to stop the rotor tilting when changing a wheel. Just makes it easier when having to hold a wheel with one hand and fit wheel bolts with the other.
The diameter of those screws is not designed to hold the rotor under full braking force as they will shear.