Those screws may actually be Japanes Industrial Standard (JIS) screws which look similar to phillips but they aren't. Getting a few JIS screwdrivers can prevent a lot of issues with messing up the head of the screw.
They are tricky. You have to make sure that the mounting screws don’t jamb up when you twist it back too. If the are still in the mirror, pull the threads all the way down and tape them in place. This will prevent the heads from coming up and chipping the mirror body. Then, grab some rubber coated work gloves 🧤 or rubberized garden gloves. This will make your hands stronger, stickier and allow you to twist the two half’s of the mirror to the desired position.
