That's so weird, my Hanna tester has been fairly accurate thus far, I use salifert, redsea, and elos. Usually all within 20 ppm difference. Which mag test kit did you get from brs? I only have salifert and red sea to test mag but never does it hit that kind of numbers... See what ever is cheaper, either send a water sample to myka or if it's cheaper and faster you can send it to me in bc and I'll test it for you too. My wife does a lot of the testing for me which I found to be better, she is a lot more careful and accurate than me.
are you still dosing probiotic by chance? How long ago did you start that system, was it prior to the crash, maybe the answer to your current issues is related to probiotic system. Did you test the freshly made salt water and see what kind of results it gives you and maybe use it to reference the results from your tank water. Keep in there bud. It will work itself out.