Ok on topic, the planets names mean nothing. The odds of a word surviving that long are virtually none as far as I can see, espescially given no other names have remained. There's nothing from my memory to suggest Spira wasn't Earth anyway, other than the obvious that shows most FF games aren't on Earth.

As for Super Nova, you might not have noticed, it's NOT REAL. The planets aren't really destroyed, it's just an illusion, the only thing that it really gives evidence of is that Jenova's still the one in control.