Hmm, interesting. Sodium benzoate is only fungistatic under acid conditions.
Potassium sorbate doesn't kill yeast. It prevents yeast reproduction. It's effectiveness is influenced by the ph of the brew.
I really doubt that you would get enough potassium sorbate in a flavoring to cause you any issues with bottle carbing. If it does anything, it would probably just slow things down a little.
And, as always, have nice day.