I'm a U.S. citizen working overseas with the U.N. Can my wife (GC holder) use the N-470 to preserve her residency while staying overseas with me?
Yes, she can, as long as she meets the other requirements of N-470.
I got my GC on March 2006 and same time I got my Reentry Permit too. I was out of States within 2 years that I worked with US Forces Yokosuka Japan for Master Labor Contractor in less than 2 years. Is this form N-470 brand new rules?
Form N-470 has existed for a long, long time. This is not a new form or rule.
Is it possible for me to work outside the US (for the same company that I work for now) while on green card? My company has an office in India, and I have to travel to India for family reasons. I was planning to work in India for some time, and then come back in about a year and half and apply for citizenship. I have lived in the US for about 3 years on my green card now.
It is possible. Look into both I-131and N-470 filing through a lawyer.