Deleting is also pretty straightforward - I would even say too much, since I occasionally deleted custom searches, just when you drop down the list of searches, swype the selected to be removed to the right, that will remove the search from the list.
Editing is not possible within Mini to my understanding. You need an Opera account and Opera desktop browser. While you have the desktop browser running, setup an Opera account. Select from File menu Synchronize. Register your account if have not got one already and login to it. Now pick your Mini and login there, too. Go to Settings and Synchronize or whatever it's called in your language. I'm having it translated here, so not sure how it is called in English or your language.
Now if you setup a search engine in the desktop browser, it shall be right away synchronized to your Mini and vice versa. Your Mini search engines will be copied to desktop browser on a computer, and you will be able to edit them in Settins > Browser > Manage search engines. Some of the default engines are not allowed to be changed sadly, but you may in example make a copy of Wikipedia in your language of choice and name it accordingly.
Same goes for bookmarks, speed dials, open tabs that are carried over from one device to another. Or actually from one Opera to another. Pretty neat. If you have a Mini and Mini beta on the same phone, they will copy also your setup to each of them, if you are logged to both of them.