I ran mine without the liner for a few months so I know what you're saying re. heat from the roof destroying the glue. There is quite a bit of heat (lots actually) coming through there as it's only thin metal at the end of the day. I also happened to notice just how much noise came through there without the liner.
So ... first stage was to dampen the noise with some heavy duty sound insulation that I happened to have kicking about, this had the added bonus of suppressing the heat as well - pretty much the same job as the original liner does - in effect becoming a new 'primary liner' if you like
Both of mine have suffered from the air bubble and in both instances it has been due to the liner and foam parting company from each other rather than the liner and foam parting company from the liner board. I imagine the fix to be more or less the same with the difference being to use a foam-backed material or not.
Full photo instructions in the thread that was hilighted earlier, click HERE
and go to page 2, the trim adhesive that I use is Trim Fix which is a temperature resistant upholstery adhesive spray and comes out like 'silly string' (other brands are available). This on its own should be enough but coupled up with insulating the roof as well should ensure no sagging ever again .. we shall see