re: drywall compound over old lath
If the gypsum core of the old drywall is exposed, then you need to replace the strength of the missing paper. Merely having paper on the back of the drywall isn't good enough. You need to have paper (or a reasonable substitute) on both sides of the gypsum to have a rigid wall surface.
Dilute white wood glue with water to make a paintable consistancy, and apply that to the bare drywall. Allow to dry and then apply strips of self adhesive fiberglass mesh drywall joint tape horizontally over the area where the paper is missing. Then, paint over those strips with the diluted white wood glue. As the glue dries, it will bond the fiberglass mesh to the gypsum core. Now do the same thing, only apply the fiberglass mesh strips vertically. And, paint over them again with the diluted white glue so that as it dries, it bonds those strips to the wall, too.
You have now replaced the strength of the missing paper with fiberglass.
Now, skim coat over that repaired wall.
If this wall is going to be in a wet area, like the shower, then you're best off removing the drywall completely and installing a product called "Dens-Shield" made by the Georgia Pacific Company or a cement board type material like Wonderboard.
Bashing my head against the walls in some of the internet's finest chat rooms.