With the Red Sea test kit you want to take the reading when it first changes. I used Instant Ocean for a lot years and never had problems; maybe should have stuck with it. I'm now using IO Reef Crystals. A dead coral is just a rock, won't hurt anything. Personally I take them out - I don't like looking at dead corals. I'd get a regular thermometer so you can double check.
I'd stick to doing the water changes and test a couple weeks before and after.
