Headliner is separating in various areas

They have a sticky spray that you can use to help with that problem. Most auto parts store should have it

I wouldn't waste time trying to spray new adhesives ontop of the rotting adhesive. The truth of the matter is that even with a new layer of adhesive, with every day that will go by more and more of the adhesive will rot, and more frequently you will see your headliner sag. You have two options really: 1) Find a way to remove the rotting adhesive and reapply a fresh coat of headliner adhesive 2) Get a do it yourself headliner replacement kit. I once got mine from https://www.headlinerfix.com Best of luck with the chevy!