As is normal, the t-top headliner is sagging. However, the car has only 23000 miles. The material is in great condition & I believe the board is also.
Has anyone attempted to carefully remove the headliner material, cleaned off the failed glue, removed the failed glue using a vacuum, a brush, or sandpaper from the board?
Then sprayed glue on the board & fabric & reapplied it?
The headliner is still installed but come September, I will be removing it to examine & to determine my next step.
Comments?

Seems like this question comes up 2 or 3 times per week. There is a search function on the forum to help you find the answer.

Definitely possible and been done many times before, It all depends on the condition of the board itself. I tried repairing mine and doing it but years of leaky t tops made it impossible.

Its more about whether the material has stretched too much to put back the way it was. My bird's had tears and we'll get back to that, but my '77 sedan deville's liner is otherwise good looking but has been sagging for years and several owners. I do some work with textiles; that much stretch over time has deformed the fabric, and fabric is all about the shape and pattern piece. So be aware you may have a larger piece of fabric than was installed in the '80s.
When I got to taking out the liner in my '88 TA, I realized the fabric in it was toast despite the good seal on the T-tops, and I wanted to install one of those ~'85 flip dial consoles like in the RCR episode. I ended up just getting some fabric in a print I liked (snow leopard minky! I KNOW) and cutting it to shape. just used some particle board from the hardware store to make a new board after the clown at my old garage swiped it & the first set of rebuilt headlight motors.
Whatever you end up doing here, much luck brother

I also remember just going over this with someone recently.

No, it's not possible. The disintegrated stuff isn't glue, it's the foam backing that makes the material, headliner fabric. The fabric is about 1/8" to 3/16" thick, because of its foam backing. When you peel the fabric off the fiberboard, the brown crumbling residue that remains glued to the fiberboard will be the remains of the foam backing. That's why the fabric face is now sagging, because it no longer has its foam backing.

The backing is dead, that's what it is; it's no more; it's ceased to be; it's tired and sagged out; it's dead fabric.

Your assessment of the removal, brushing and vacuuming is accurate, and the fiberboard will probably still be perfectly fine after that prep and ready for new fabric. But you'll have to install all new fabric. You might not believe it at the moment, but once you begin removing it, then you'll understand.

Grey and beige are the common colors, and they're usually available at local fabric and hobby stores, and it's not expensive.

I understand what u are saying. The colour of the interior on mt T/A is code 22C - blue. Been checking the various sites for that blue (called also called dark blue)
The search will continue. I hope to find a supplier here in Windsor, On.

I didn't mean for that to sound so harsh. I threw in the Monty Python reference to keep it light lol.

I just did a web search and found many, many sources for the stuff. The images never look exactly right, so it's hard to know if it's the right stuff until you actually see it, but the products in this link are likely to be correct, since the vendor is one of ACC's auto carpet representatives. When you click on the bold, "view headliner colors," you'll see two full ranges of colors, including the blues you're looking for.
Surcolor Replacement Headliner Kits (automotiveinteriors.com)