here is a dumb question, but is it possible to remove the dash completely with the windshield still in place?
Yes, but it gets very hard to get at the attachment points that are under the windshield rubber. You need to use something to pry up the rubber lip while you remove (or install) the fasteners. I put a dash into a car that had the windshield already installed. It was a pain and took all afternoon but I was able to do it.
When I installed the dash, I enlarged the screw holes in the firewall flange to allow the screws to slip through. Then I put clip nuts on the dash flange, and installed the screws from the bottom. The windshield is not a factor anymore. The screws can be accessed with everything except the a/c blower in place.