Can citric acid be used as a seasoning?

Answer 1

What's happening is that the citric acid denatures some proteins. Heat also denatures proteins, so it's similar to cooking, but since no (or, at least, very very little) heat is involved, it's not exactly the same (it won't necessarily kill parasites or bacteria, for example, and only those portions that the citric acid penetrates to will be affected at all).

Answer 2

Yes.

Citric acid is the same acid often found in citrus fruits, like lemons, limes and Oranges.

Pure citric acid has a generic sour flavor, but yes, I suppose it could be used to flavor food.