Yes, "xgui show bans" would show the whole bans tab, presuming they have permission to see it.
I'm not quite sure if I fully understand your second question, but there is a now a search box on the bans tab you can type the admins name/SteamID in to show only who they have banned (no way to automate that at the moment, I'm afraid)
Yes, you can PM via GUI-- just use something like this when send is clicked: RunConsoleCommand( "ulx", "psay", <playername>, textbox:GetValue() )