How high does the KA chart go? That doesn't seem likely with that PH and Calcium level.
Nitrates don't just go away on their own, they need to be consumed and/or removed. If you have no provision for removing them, like water changes, then they will continue to build up.
How old are the test kits, and are your sure you've followed the directions? I once got the directions for two different tests mixed up and scared my self to death.
12dKH is possible, almost normal. I wouldn't worry about it but stop adding buffer if you are.
200ppm nitrate isn't impossible, but not likely. Mix up some fresh saltwater using pure water and a good salt mix and test for Nitrate to validate your test kit.
Guy, My nitrates are at about 120. The funny thig is I did a 20% water change this morning and it didnt even phase either the nitrate or KH readings! What is ideal KH for a reef tank? I had aways had around 6 or 100 ppm.