am sorry dear, but I dont think that swimming in a pool once or twice will destroy your hair, which is so healthy and beautiful btw. I posted coz i used to swim 4 times a week, and suffered from serious chlorine build up and my hair was really dry and damaged, but this was over a period of 4 months. My hair improved only after i chopped off 3 inches and started doing what I outlined above.
For your question of how many times a week you can use a chelating shampoo, it depends on you. How many times a week do you normally shampoo your hair? you can chelate as often as you shampoo. So if you normally shampoo more than once a week, then the answer is yes, and vice versa.