People have been creating names by back-spelling both in English-speaking countries and in Latin America for many years. It would be hard to say what person first thought of creating Nevaeh. There were 8 girls named Nevaeh born in the United States in the year 1999. By pure chance I discovered that one of these was born in Nebraska and was able to interview the mother. She told me she had thought it up herself. When she was a teenager she was a fan of the novels by V. C. Andrews that featured a heroine named Heaven, and she was thinking of that character when she reversed Heaven into Nevaeh for her daughter.
The boom in girls named Nevaeh is because of its use by the American rock singer Sonny Sandoval. His daughter Nevaeh was born in March 2000. In the fall of 2000 he was featured on the MTV cable TV program Cribs, where he showed off his infant daughter and explained her name. I have not been able to find out yet whether Mr. Sandoval and his wife independently invented the name or whether they heard it somewhere else. But the publicity given their daughter obviously is what originally spread the idea. Most of the examples of girls I know of who were born in 2000 or 2001 named Nevaeh were directly inspired by Sandoval's daughter. After that the name was discussed so much on the Internet that it's probable that many of the parents using it after 2002 were not directly inspired by Sandoval, however.