Well, about offline availability, a large number of instant answers(spices and fatheads that are) use external APIs or indexed databases from websites, so they can't work offline.
DDG does have official(and unofficial) browser extensions and apps for iOS/Android.
> Well, about offline availability, a large number of instant answers [...] can't work offline
Sure, but there are a large number of instant answers that can and do work offline because they're simple, static tables, or are self-contained—existing only to apply transformations on the input (e.g., cheatsheets, natural language unit conversions, and calculations).
> DDG does have official(and unofficial) browser extensions and apps for iOS/Android
A browser extension that just sends the query the same as it would if you hit their homepage is in the "what's the point?" category, just like mobile sites that nag you to install their app when all it does is show you the same content that is (or could be) on the mobile site itself. The "is a browser extension" is not the interesting part. "Doesn't send data to a third party" and "can operate without being connected to the network" are.
DDG does have official(and unofficial) browser extensions and apps for iOS/Android.