- Enter for newline and Shift-Enter for new entry (and Alt/Cmd-Shift-Enter for new child)?

This would retain your multiline discoverability. I would be careful with Alt, as it makes CrushPaper awkward to use on Macs, where Alt+s both sends the keystroke and types 'ß' (German eszett) and similar issues abide for other Alt combinations. Since you have so many Alt shortcuts, it might be worth mapping them to equivalent Cmd shortcuts on Mac, as web apps commonly do.

- OmniFocus/Outliner on Mac use Escape to finish editing and Enter always makes a new entry. Escape might make an easier-to-hit-reliably edit key than F2 (which can be hard to hit without looking/fumbling, even for a touch typist).