It is true that you must have had a continuous year of physical presence in the U.S., but you must have already fulfilled that requirement to get the first N-470.
It is not true that you have to file the N-470 before you leave the U.S. You can file the N-470 after leaving the U.S. and even after you have started the employment abroad, but it has to be filed within one year of departure from the U.S.
You can therefore file the second N-470 without returning to the U.S. if you have been out of the U.S. for less than a year.
This is only a preliminary response to an inquiry, based on incomplete information. Helpful advice and counsel can only be provided after you have discussed your case with Mr. Bach, he is familiar with all of the circumstances of your case, and the advice is provided in writing. Any opinion provided here should not be relied upon for any purpose. No attorney/client relationship is formed until you have formally retained our law firm to represent you. We are not responsible for any action or lack of action you take based on this preliminary response.
You have to file N-470 before you leave the U.S., and you must prove that you had spent one year continuously in the U.S. as a permanent resident.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. Consult with a qualified attorney before making any legal decisions. Gen Kimura, (832) 247-6932.