Having a zero reading and a .02 reading makes sense due to the test kits limits. If you fear it maybe wrong you can double check it with another. I think it sounds spot on as you think you have too low of a level. I would say keep using it and keep up with the trend of the kit as the trend is better to see than nothing.
As for the kalk if it smells funny or is not pure white ditch it and get some new stuff.
Seawater over synthetic is always a great choice if collected correctly. Unfortunately with you working on a fine line with a finding a perfect balance you would have to pick one of the two for stability. Fine tuning levels like this is a daunting task, but is at the pinnacle of reef keeping. Keep up the good work.