If I'm understanding you correctly, it sounds like you want to run two separate sites (one on the main domain and the other on a subdomain) from the same database. You can do that by creating a second install of WordPress in that subdomain and configuring it to pull information from the same database (where your content is stored). Just keep in mind that any changes you make to the actual content will apply to both sites as they will both be pulling from the same database.
The mobile plugins will allow you to choose a different theme for your mobile site. Some even come with their own mobile themes, which is the best way to to provide your users with a good mobile site experience.
Hope this helps!