Hey all!
I'm creating my first Mystic deck (with Agnes Baker).  I decided I needed at least one card (two copies) to deal with getting engaged by a really dangerous enemy.  This could be to evade altogether, OR to survive a couple of turns of steadily attacking away to deplete their health.  The thing is I can't decide between Blinding Light and Hypnotic Gaze.  Does anyone whose played extensively using a Mystic investigator have any advice?  I loathe the idea of wasting XP swapping out level zero cards because I chose poorly at the get go.

Continuing with the same investigator is a bit more official than most as variants go, but the actual standard game rule is that you start a new investigator. The section at the end of Night of the Zealot is pretty clear about that.

Hey man, do whatever you want.  It's your game.
 
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Just joking.  I see no reason why you couldn't stack two copies of Charisma.  If you have the XP, go crazy.
As to the question about campaigns, I don't have my rule book near me, but I think it addresses this question.  I think the answer is something like 'the expectation is that you'll start over each time you start a new campaign, but if you want to continue, you can...  you just do so with both the good stuff (XP cards) and bad stuff (traumas).

"Do whatever you want" is actually the "official rules" answer.  The final section of the Night of the Zealot campaign guide says you normally start new, but can continue if you want.  So while the official rules are to start a new investigator, continuing with the same one is an officially-endorsed alternative.  So in other words, "hey man do whatever you want, it's your game"
For Charisma, nothing stops you from buying two copies and gaining the benefit from both.