Using Webmaster Mode (Online or Offline?)
 

For the Webmaster Mode, it says:

"Webmaster mode is based on sessions: it applies only to the computer it has been set up on. It doesn't affect the website itself;"

Does this mean I could edit on-the-fly? In other words, I could keep my site online for my customers to shop while I do the editing? All I am planning to do is customize the English labels. TIA.

Yes, you can edit on the fly. The only way a customer will see the debug console is if you enable it in General Settings, which you don't need to do to use webmaster mode.