I dont think 15 dkh is high enough to cause a tank crash. I accidentally pushed my alkalinity up to 22 dkh once for several days and all I lost was one acropora.
I havent used salifert in the last two years. I didnt know they could test pH and came with a referance sample. I thought only Seachem did that?
To bring your alkalinity down just shut off your calcium reactor and kalkwasser reactor for a few days and it should come down quickly on its own. Test daily to make sure it doesnt drop too low. To speed up the process do a water change. When you start them up again you may want to lessen the flow through the reactor some more.
Another way you can check the accuracy of a test kit is to test some newly mixed saltwater. Your results should be similar to these
http://reefcentral.com/forums/showthread.php?t=1714505