She actually said dragons can live longer than planets as apposed to a specific time period so it wasn't exaggerating a number but a statement. Like you said I guess they are immortal (from an age perspective) and on a similar power level to the gods of that world. I mean Lucoa WAS a godess.
I'm fairly sure even the manga won't really go into this stuff TOO much - will probably not really find out all about the war etc, and it will end with them all still living on earth. Given how short humans live by comparison, on a human timescale I bet the war moves VERY slowly - and it dosn't seem to be that sort of manga.
As for Tooru's power level - one thing to remember is she is still quite badly wounded. One of the things she was worried might make her return was fully recovering. They haven't really mentioned it in the anime bar her holding her side once or twice. I bet She would have dealt with Elma easier otherwise. Also in that fight she was hampered by being in a human city and trying not to get people killed while Elma didn't seem to care.