For a windshield, I would use masking tape to secure it to the body.
I'm big on mock-ups. By the time I finish a model, much of it has been together and apart again dozens of times. It goes a long way to reducing the number of issues at final assembly, though I must say, it does not eliminate them. There is almost always something significant that goes together very well in a mock-up but, maddingly, still doesn't want to fit smoothly at final assembly. Sometimes I wonder if I would be better off not doing frequent mock-ups... at least there would be an excuse for things not fitting at final assembly.