Concerning the editor I suggest to take a look at the german app “Wortkraft” (they also have “Schreibkraft” and “Textkraft” which seem to differ only in some features) by developers Infovole. Sorry, I don’t know how to link to the appstore.
I their extra keyboard row they have cool navigation buttons, including navigating by letter, word or paragraph and also up and down a row. They also have a good solution for text selection. The selection button selects the word next to the cursor, then there are two buttons to extend the selection wordwise to the left or right. The coolest feature, which is a novelty to me, are capitalize and decapitalize buttons. They affect the word next to the cursor and change its first letter. This is practical in german since we have a lot of capitalized words (all nouns). And I keep getting messed up with the shiftbutton, having words capitalized which shouldn’t and the other way around. It’s also good to undo auto-capitalizing after a point that indicates an abbreviation, not the end of a sentence.
I don’t know if features like these are patented and wether they fit in your layout, but maybe it can be a source of inspiration.
I find the iPad keyboard much better than I had expected, but editing/writing is still clumsy and good custom keys can help a lot.
Chears
Robert