This will be changed significantly in 12.04. As additional answer to question 2
Three out of the bottom 4 icons can be adjusted as noted by changing the default - internet browser;email;music, the photo viewer choice is coded in the source
If your preferred is not listed in System Settings ... you could probably track down in gconf-editor but no need. Each of the 3 is set from 1 specific mimetype, so it can be altered in ~/.local/share/applications/mimeapps.list
in the [Default Applications]
section.
They are
browser - x-scheme-handler/http=
email - x-scheme-handler/mailto=
music - audio/x-vorbis+ogg=
Example here
[Default Applications]
x-scheme-handler/http=google-chrome.desktop
x-scheme-handler/mailto=thunderbird.desktop
audio/x-vorbis+ogg=vlc.desktop
Generally speaking most apps for these types would register when installed though a quirk in gnome3 is that if the app's .desktop doesn't have an Exec= line that ends with %<letter>
it won't show as a default choice so that's something else to check