Hi Xavier,
I have tried making the changes according to your instructions.
However, after saving the changes in the backend's order / invoice file, I found that the Invoice & Shipping Invoice were displayed as blank pages. (Screen Shot A & Screen Shot 
When I changed the script back to the original and saved the file, the same blank pages are displayed.
It's only when I clicked on the trash bin icon under the "Remove Customization" column, that the Invoice & Shipping Invoice are displayed in their original form.
Did I miss out something?
Regards.
Kevin