Just found an interesting work-around!
(1) Click the menu button and go to "Appearance", then click "Buttons", then click "Search". Drag-and-drop Startpage onto the top toolbar AS ITS OWN SEARCHBAR.
(2) Remove the default searchbar from the toolbar altogether.
(3) Close Opera.
(4) Go to search.ini and simply delete the entire paragraph pertaining to Google:
[Search Engine 1]
UNIQUEID=7A8CADE6677811DDBA4B5E9D55D89593
Name=
URL=http://www.google.com/search?q=%s&sourceid=opera&ie=utf-8&oe=utf-8&channel=suggest
ICON=http://redir.opera.com/favicons/google/favicon.ico
Suggest Protocol=JSON
Suggest URL=http://clients1.google.com/complete/search?q={SearchTerm}&client={Client}&hl=en
Query=
Key=g
Is post=0
Has endseparator=0
Encoding=utf-8
Search Type=0
Verbtext=0
Position=-1
Nameid=17171
UseTLD=1
(5) Restart Opera. Address bar will not work for searches. Now simply use the Startpage searchbar you created before!