You are expecting way too much from the spray gun. It will not shoot glue like paint, that's not even remotely possible. The glue should look like really bad orange peel when sprayed. Make sure it has plenty of air and plenty of glue going through it, and spray it at a distance of about two feet. Yes, I said two feet.
You can get the back corners if you stretch the headliner material enough. Worst case scenario you have to cut an upside down "V" in those two corners and butt the headliner fabric together. I think you'll be surprised when you see it's not going to be as bad as you think.
No one lives forever, the trick is creating something that will.