I hadn't thought of that. Yes, if Dee kills the elders on Danu Talis, there wouldn't be anyone to grant him immortality in the first place, so he would never exist, at least in that strand of time.
And to just bring up what someone just said, why didn't the Flamels simply use the spell which infers immortality on themselves instead of brewing the potion all the time. I mean, Marethyu showed them how to read it. And don't anyone tell me that in 700 years he didn't get a little curious and look at the rest of the Codex. Also, in case anyone thinks it, an Elder can't be the only one to say the spell, as elders can't touch the Codex.