Robert...
The EDDs won't update automatically. You have to open each "container" EDD and save it. The auto-update of an inset won't ripple through nested insets. After that you have to open each root EDD and import that into the appropriate template.
When you say "how to edit the commonElements file" .. I'm assuming that you know that it's a regular (structured) FM file that you can just open in FM and edit. You're looking for someone to tell you the actual syntax and location for the context rule to add to that file? Sorry, but I'm not going to sign up for that right now .. it's not all that hard, but it's a lot to explain. You should probably do some testing and experimentation on a simple (non-inset-based) EDD and structure app, so you understand how EDDs, element definitions, and context rules work. On this page are the Structure Application Developer's Guide and Reference PDFs ..
http://www.adobe.com/devnet/framemaker.html
About half way down in the "Structure Application Development" section. Play with this stuff for a bit and it'll start to make sense.
Cheers,
…scott