The problem with an LFS
testing your water is that they can not tell if your tank cycled or never started, unless they were testing during the cycle. At the very least, at the end if your cycle, you should have high nitrates and need to do a partial water change. "Perfect water" at the end seems questionable.
What's in your tank now? Seems odd to ask about fish potentially dying if everything is looking good.
In answer to the question though...if fish die, you would likely keep your water, but it would depend on what killed them.