2. By data linking, did you mean data binding? This is just a means of linking properties and variables together, so that when ones changes, so does the other.
3. The mxml gets complied into actionscript before getting complied into actionscript bytecode (in an swf). If you add the '-keep-generated-actionscript' tag to your mxml compiler, you can look at all the actionscript that gets written for you from your mxml.
4. An SWC can be used as a runtime shared library, or statically. If you use it as a RSL, then the components don't get compiled into your swf, but are loaded in at runtime, instead of all getting included in your swf at compile tim