Setting up a mobile site
I think you are on the right track, but I think doing this as a true multi-site is overkill, and potentially the wrong idea.

Ideally, you want your desktop site and mobile site to share the same content.  If you are not going to go down the route of just using mobile stylesheets, then I suggest using Domain Access to create the mobile site.

You can use this module to set up your mobile domain, and then create your content accessibility rules for what is visible where.  When used in combination with Panels, you have great flexibility.

Mobile Tools let you choose your theme for a user and also shows up in panel selection rules, and domain tools lets you show/hide content for a particular user.

Note the issue log for Mobile Tools, though.  I found a bug with how the ctools selection rules work, and have a patch on the issue that hasbn't been committed or commented on.