But at night when you are asleep your metabolism is lowest isn't it? So therefore your body would need to break down what food is left in your stomach before hitting down time wouldn't it?
I have never heard that you shouldn't eat salad in the evening. If the theory is true, that it takes less effort to break down cooked veggies, I would think it better to eat the raw ones in the evening to keep the metabolism higher for longer periods.
Yes, cooking does cause some, not all, of the nutrients in food to be lost. Steaming them is much better and preserves most of the nutrients. Keeping the majority of your food as close to raw as possible preserves the most nutrients; of course, this does not include things like meat and other items that need to be cooked prior to consumption.