I used the trick with altering the defaultpartnercontent.json file to get bing back, but today bing got thrown out and replaced by DuckDuckGo again, so I had to overwrite the .json file again...
It's working again and for now it's ok, but if this keeps going like that and Opera won't let me decide what search engine I want to use, I have to change the browser. This would be a sad thing because I've been using Opera for a very long time so far, always trying out the others and Opera always was the one which suited me the most, but the fact that it just changes my settings without asking is absolutely unacceptable.
Not the way to go Opera developers...