I think I got it.

My Button MovieClip was linking to a class in the the same src folder as the rest of my actionscript code in the Flex actionscript project. (The as3 classpath was set to "../", from the assets folder where the .fla file lives, one level up to src )

When Flash compiled, the class was found and I got in the .swc file. That is why I got the symbol traced from flash. When Flex compiled, the class was found in the src directory, so a new version of the same class was compiled, although it already existed in the swc file. Since flex found the .as file, but wouldn't know I had it linked to a symbol in the Flash IDE ( I don't think it can do that ), Flex created its version of the class, but since no symbol was linked to that class, the child of Button I was tracing was null.

I guess using packages and keeping the fla linked classes separate so Flex wouldn't compile the same class twice.

The weird thing is I didn't get any warning or error, so I assumed everything was fine.

A quick and not so dirty workaround was to delete the physical Button class from the filesystem, so that Flash could generate it's version on compile time and have references to the children of the Button symbol. Then in Flex I cread a class that extends Button and added the code I needed. Since the generated class got compiled in the .swc file, Flex could see that and I had no problem extending.

That was it! Bob's you're uncle!

I assumes that what happens, I am not 100% sure. It's something my intuition takes for granted through observation, so I might be technically wrong. If anyone has any correction they're welcome.

You can publish your file as a .swf and use the SWFLoader to load it, just remember the paths are relative to your run/debug directories.

My understanding is swc files are generally library files, rather than visual components, because you want the libraries included at compile time (swc) rather than at runtime (swf).

        var loader = new SWFLoader();

         // set autoload to false just so we KNOW when it loads. Also prevents
         // us from accidently loading it twice by calling load() later
         loader.autoLoad = false;
         loader.addEventListener(Event.INIT, function (nt:Event) {
             // Remove event listener so can be garbage collected
             event.currentTarget.removeEventListener(event.type, arguments.callee);  
             // note we can the loader, rather than
             // loader.content
             someIVisualElementContainer.addElement(loader);

         });

         // relative to bin-debug or bin-release
         loader.source = "movie.swf";

        loader.load();