Sounds like you have some home repair skills, so this should not be a huge problem to correct. I have never heard of injecting glue into the panels. I'm afraid you may just make the problem worse by increasing the weight of the panels. I would also expect the glue to discolor the ceiling and possibly make it difficult to paint over
As you mentioned, very slight sags can be corrected by wetting the panel with a spray bottle, pushing up and holding for several days. This has worked very well for me several times. But not every time. This depends on how much sag, the composition of the ceiling panels and the spacing of the truss-rafters. While there is no guarantee that it will work in your situation it is the easiest solution, so I would try it first.
For the best alternative recommendations read Manufactured Home Ceilings - Repairing or Rebuilding?
in our article archive. It is written by Mark Bower the author of "The Manual for Manufactured/Mobile Home Repair and Upgrade"
and offers several solutions.