Does the dash have to be in to install windshield
No they don't. Many of us have PURPOSELY reversed the clips for the screws so that the dash can be REMOVED OR INSTALLED with the glass in place.
You probably need slightly shorter screws as they will impact the rubber.
I RECOMMEND THIS METHOD
The reason the dash was installed first, at the factory, was that it was simply easier. They could reach through the opening someone could get it positioned, and buzz the screws in