Those are some good points, but I don't think it is fair to assume that because the lettuce was cooked that it has LOST nutrients. I once read about a study on human food stating that cooking vegetables sometimes releases nutrients that are otherwise unavailable (kinda like the principle behind coprophagy). Of course it matters what you are cooking and for how long. As long as you use a high quality lettuce like green leaf, collards or dandelions, I doubt there would be any deficiencies. Although I am sure EdK could clear this up for us.
As far as using pond water, I just don't care to take the risk but you are right that you should use your best judgment to determine the benefit / risk ratio.