ActionScript 3.0 :: Subclassing - Library Items And GetDefinitionByName?

I've recently inherited an AIR project, which is basically a widget that displays product images and static content (contact details etc) on the desktop.The original project was developed in CS4/AS3, and has been published and works fine. Since then, the original developer has gone overseas and I've been hired to make new versions of this widget, UI reskins, different products, etc. I work in CS3, so I've had the original Flash authoring file saved down, gotten Flash CS3 to be able to publish for the AIR runtime, and found the relevant AIR classes that I need. This has all gone smoothly.

The problem I'm having, is that there seems to be some strange way that the original developer has linked the library items to their relevant classes, and I'm trying to unravel this.I'm going to use the clip called windowApplication as my example.In the library, there is a movie clip symbol called 'windowApplication'. In its linkage settings, its class is set to 'windowApplication', and its base class 'com.aca.windows.Application'.The Application class extends the MovieClip class.However, when I test the app, I get compiler errors

'5000: The class 'com.aca.windows.Application' must subclass 'flash.display.MovieClip' since it is linked to a library symbol of that type.'
'1151: A conflict exists with definition SectionBackground in namespace internal.'
(SectionBackground is the first variable declaration in the Application class. Commenting it out makes this error change to the second variable declaration).

I can remove this error by changing the class of the 'windowApplication' symbol to 'Application' and setting the base class to 'flash.display.MovieClip', however that doesn't seem to instantiate the class properly. The symbol appears onscreen, but none of its instantiating functions are called.

The windowApplication object is instantiated using a getDefinitionByName call. I've had a google and looked around various forums, and I've seen suggestions that classes used in getDefinitionByName need to also have a fake instantiation so that they're compiled in Flash. I've tried this but it makes no difference.It seems to me that the getDefinitionByName call is working properly, as it does bring up the correct symbol - but only when the base class is flash.display.MovieClip.

I had another movie clip, called sectionMain, that also had this problem. Its original class was 'sectionMain' and its base class is 'com.aca.sections.Main'. It, too, threw a compiler error 5000. I changed the symbol's class to 'Main' and the base class to 'flash.display.MovieClip', when testing the app, this brought up the symbol. When I changed it back to the original settings, it worked fine, the object was called properly and I could interact with it. However, when the errors with the Application class are thrown, I also get the 5000 error for the Main class - without having changed anything.When I change the class of the windowApplication symbol to 'Application' instead of 'windowApplication', I get a warning that Flash can't find the class and will create one at runtime.