Is it a vinyl headliner or cloth. Reason I ask is because if it's cloth, and you accidentally sprayed adhesive on that last structural rib, the adhesive may be seeping through the cloth, discoloring it. Don't think there's no fix for that. If it is vinyl, it probably won't seep thru. Try wiping it with a soft cloth soaked in denatured alcohol. If that don't work, try hot water and detergent with bleach, and dry it.
There doesn't need to be any glue on that last structural rib (in case you ever redo another one). I used a brush-on can of upholstery cement and brushed it into the bottom lip of the window channel (before the back window glass was installed). You can hold the headliner in place while the glue dries overnight using a bunch of binder clips. I think I had four boxes full of clips (thats 16 each).
The factory actually folded the headliner material around to the top lip of the window channel and glued it on there as well for extra hold. I did that too. You can apply the glue to both sides at one time, pull the headliner tight, wrap the headliner around that window opening and insert the binder clips to hold it in one fail swoop.