First i want to thank all those that are answering my many questions, just getting back into the game after at least a decade. Just got a JLU booster and want to make sure I got this right'

Lex Luthor gets replaced with the Flash when KOed, so is the +10 added to the opposing players points for KOing the figure? If you don't play for points then its a moot point, correct?

I think the (+10) before the power means you have to pay an additional 10 points when you build your force, if you want to be able to use that power. If flash is then KO'd your opponent would score victory points for the higher point cost of the two figures. In this case 40 points for Lex Luthor.

Thank you, so can keep switching then when KOed, the power doesn't say once per game

You can only do it once because you can’t pay the 10 points for Flash to have that power since he is brought in from outside the game.

Gotcha Thank you, They added so much!

Replacement is from the Sideline, not outside the game unless the effect says otherwise.

Since Lex isn't KOed, the 10 points isn't scored when he's replaced, nor should it transfer to Flash. I believe the only way to score the 10 points is to wipe your opponent.

So is the optional trait only once per game?

Lex is replaced after the trait is used, so yes, you're only going to be used once.

The clarification in the Comp that you can only pay once is because some optional traits are things like Sword Bearer in X of Swords, and people would absolutely pay for that multiple times if there wasn't a rule against it.