The bit about Serenity the Elder being an avatar is news to me too and I thought I was reasonably familiar with the unbutchered Anime....
If you want something that shows why Usagi might qualify as a goddess I suggest the ending of the manga in paticular. She does god-like things like raising the dead and other assorted miracles before that, but during her final confrontation with Chaos and when the true nature of Chibi-Chibi is revealed you get a good idea of the true scope of Sailormoon's potential. At the end Mamoru even says something along those lines when he contrast his and the senshi's expected longlevity with Sailormoons true immortality.
If you want something that shows why Usagi might qualify as a goddess I suggest the ending of the manga in paticular. She does god-like things like raising the dead and other assorted miracles before that, but during her final confrontation with Chaos and when the true nature of Chibi-Chibi is revealed you get a good idea of the true scope of Sailormoon's potential. At the end Mamoru even says something along those lines when he contrast his and the senshi's expected longlevity with Sailormoons true immortality.