Yeah, my Florida and Texas 2006 had that gloopy broken-down glue residue, as well.
I scraped as much as I could get off with a credit card and nitrile gloves. It still didn't want to adhere to the spray on headliner adhesive I used.
I might suggest spraying or soaking in Goo Gone to really get that stuff off all surface areas, if you are going to spray on adhesive.
I did a crappy job of cutting my headliner replacement, so the hatch dome light exposes cut marks at the corners. I keep thinking that I will have it all taken apart and professionally redone, someday. It might be worth ot to consult a professional auto upholsterer.