Can I glue a second layer of drywall?
A good drywall adhesive has much more shear strength than screws - and this is just when your glue is on the framing.  So if you are doubling up the drywall gluing makes a ton of sense.  

Just recently my drywall guys started using glue and I can say this, I hope I never have to demo these places because the drywall is ATTACHED.  

All this being said the glue holds to the back paper of the drywall.  What you are saying is that behind the drywall you don't have a solid base... then the glue won't work.  

Also I am going to assume that crumbling drywall means moisture.  While most glues will be fine with a little moisture, the back paper of your drywall will not.  

Then we have to think about heavy tiles.  Heavy tiles do not go on drywall.  They go on a backer board.  

You need to take out crumbling drywall, then install 1/2 inch Hardiboard, then install tiles.  There is no reason in the world that you need exterior access to remove interior drywall.

Recap:


general purpose Liquid Nails is not a drywall adhesive
approved adhesives are just as good or better than screwing - but not applicable to you
I wouldn't install drywall when you have an exterior moisture issue 
drywall can't be used for heavy tiles
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