Here's an update in case someone happens across this thread on a search.
This is what is observed (on Mac nw.js) alltogether:
C2's Keyboard.LastKeyCode is incorrect for numpad keys
browser.execjs("event.code") is correct for numpad keys
browser.execjs("event.keyCode") is incorrect for numpad keys.
Lastly, I can also reproduce this on Mac outside of nw.js using the latest version of Chrome (62.0.3202.75)