Opera 12 has ability to disable access of specific extensions to https sites and/or private tabs (by default access to https sites is enabled and to private tabs is disabled). May be there is a hope that they implement it in Blink based Opera, but now they only have ability to disable access of specific extensions to private windows and have no private tabs at all.
Opera 12 is ancient history. Opera 18 is now basically Chrome without the ability to set a custom search engine as default. Even with sqlite hacks, there's no way to set DDG as default on the stable release versions.