> what would you call an exterior hallway?

I'm not sure, they are common in blocks of council flats built in the fifies and sixties. I remember the ones in Moss Side in Manchester being built and announced as the next big thing - "terraces in the sky". Many of those have since been demolished. In general, I'd probably call them balconies, even if they were not just individual balconies but connecting several flats.

In the case of the Barbican Centre in London, once regarded as a fifties-designed carbunkle and now a listed building, they are called "walkways". The walkways connect the flats but some are accessible to the public. Thinking about it, maybe only the ones with public access are called walkways.

(The Barbican) ... as a model for high-density urban living of the kind we are all being urged to consider today, it has few peers. But as its name implies, it is a fortress. It does not weave itself into the surrounding cityscape at all. This is because it was predicated upon a complete high-level pedestrian walkway system extending throughout the City: a system that was pursued vigorously for some years, and then gradually forgotten. The surviving fragments of the walkway system are in their way as evocative Roman city wall that was carefully revealed during construction. The City has now gone in another direction entirely: instead of separating pedestrians and cars, it actively discourages car use, with great success. The fact that this change of heart came about as a result of terrorist bombings does not alter the effectiveness of the outcome.
http://hughpearman.com/articles2/barbican2.html

> is catwalks the correct term for the scaffolding in a theater? or is it a misused term... (that, is, are catwalks something specific, and i am misusing the term?)

A catwalk, in my experience, is the term for a built-in element of a theatre (usually a fairly large theatre) giving access to the lighting grid. The one that I used to use, wasn't big enough to walk on - you had to move along on all fours. I'd assumed that was the reference to a cat. There was a place on the catwalk for the follow spot operators to sit. When I was involved in lighting, the term wouldn't have been used for a temporary scaffolding arrangement. The thing that is moved around to access high level lighting was called a talloscope. http://www.schoolshows.demon.co.uk/resources/technical/gloss7.htm

In terms of the use of a catwalk by models, isn't it because they are notoriously "catty"?