The Twenty Eleven theme does not show a sidebar on single post pages. To the best of my knowledge, there's no way around that on WordPress.com.
I suggest checking out some other themes. Look through the demos for a theme that you like and shows the sidebar on single post pages: http://theme.wordpress.com
As for the widgets displaying on mobile devices (iPad, iPhone, Android, etc...) that'll depend on your mobile theme settings.
You can turn off the WordPress.com mobile theme by going to Appearance > Mobile and changing Enable Mobile Theme to No. This will make mobile devices load up whatever theme you're using, and assuming that theme displays widgets on mobile devices, you should be set.
Hope that all makes sense. :)