preservative to use in falafel sauce
As you're already using lemon (I assume the juice), and considering the tart/sour taste of citric acid would pair well with your ingredients; I would go for citric acid.

However food safety, especially when you're planning to commercially sell these products, is not something you should get direct advice from strangers on the Internet...

I wouldn't follow the guidance for the amounts and the processes mentioned here (or anywhere else). You really need professional opinion on creating shelf-stable food products. And you should be following professional food preservation techniques.