What is up with this wall, and what is the right way to hang something on it?
Given that it's an exterior wall you've probably hit brick.

This would explain both why you've not found a stud - there aren't any, and why the nail only goes in 1/2" - 1" - you've hit the bricks.

Given that the plaster is crumbling, the simplest solution would be to drill a hole with a masonry drill and use a screw (or screw hook).

If you want to hang a lot of things on this wall then it might be a better idea to install some sort of rail - either a traditional picture rail or a more modern alternative and hang the pictures etc. off that.