I find the "Look Up" command useful in the desktop version and now I find myself wanting to use it on mobile. How can I do this (I may have missed something obvious) or how can I most conveniently mimic that functionality?
The easy (and only) way is to turn on the Automatic Word Lookup setting in the AA menu. It's not as complete as the desktop and can get in your way absent OL preferences, but works offline, if you have the needed volumes downloaded. Quite quick.
but works offline, if you have the needed volumes downloaded
Well... It depends. There is some offline lookup supported.
The easy (and only) way is to turn on the Automatic Word Lookup setting in the AA menu.
That is both disappointing and unfortunate.