The main character describes her life on an island:「そこで、なんてことないあたしは暮らしている。」 It's translated as "[on this island] I lived my ordinary life."
I don't think -ない can qualify anything other than a noun, so なんてことない must be qualilfying あたし. Does she say something like "[...] the completely ordinary me lived my life"?
Also, I couldn't find the meaning of "なんてことない" as "ordinary" in a dictionary; could someone link a good definition?
I think your parsing is correct. Here なんてこと ≒ a surprise thing and ない negates なんてこと.
As you mentioned なんてことない modifying あたし, so the structure of the sentence is "あたし [which is not a surprising thing : なんてことない] lives on the island".
It is not so obvious that "あたし" is an average person just on the island or an average person everywhere. Anyway, "あたし" is type of an ordinary person. So, your translation "ordinary me lived my life" sounds more closer to me than to "I lived my ordinary life".
なんてことない in the thesaurus dictionary explains its basic meaning is
騒ぐ程の内容ではないという趣旨の表現
Again, あたし is not a type of getting really paid attention to in the environment (i.e. on the island).
