If a recipe calls for 12 grams kosher salt can I substitute 12 grams sea salt?

Yes since it's the same weight and not a volumetric measurement.

Yes! You can always substitute salts by weight. You only get into trouble if you try to substitute by volume, as different salts can have very different densities: http://www.seriouseats.com/2013/03/ask-the-food-lab-do-i-need-to-use-kosher-salt.html