So if KL1505 is not listed there it would mean that it did operate, so the aircraft must have been in NWI. Unfortunately it seems impossible to get the registration of the actual aeroplane (since it's not listed as departed on schiphol.nl), otherwise it might have been possible to track what flights it operated later that day.

How many other passengers were supposed to be on KL1506? Would it have been a particularly empty flight?

You can always submit a claim and expect the response stating it was because of weather and then request evidence that it was due to weather.

I had a case a few years ago with LH that I handed to EU Claim and once it made it to court they finally provided evidence from their log book that they had been asked to cancel the flight by ATC and then that was the end of the story.