Return it. API isn't known for accuracy...even the starter liquid tests that we use aren't that accurate, but they do the job we are asking them to do. Some of the best test kits are made by Salifert and Red Sea. I'm personally a big fan of the Hanna Checkers.
For Phosguard, I kept mine running in a reactor for about 3 weeks. I don't recall how long they stated that it lasts for as I have read it doesn't work longer than 14 days. Check the container to see their recommendation.
Matrix Carbon, is there any GFO
or anything in it? If not, it will not be a good replacement media for Phosguard as carbon doesn't remove phosphates.