I personally don't own N9 but I'm assuming because your device was from Switzerland, it could be possible that instead of Navifirm offering you a full blown version of Switzerland's image file, you only needed to download the updates whilst retaining the same (as per preinstalled) base.

In other words its similar to delta upgrades where instead of downloading a big file for each locale when you only needed to update your device to the same version as provided from that country you only needed to download the updated portion. Whereas if you were to download a completely different locale you will need to download the entire thing probably because its setup could be quite different.

I do not know about erasing personal contents and settings from previous owner though I'm sure flashing using Navifirm should be universal shouldn't it? if not maybe you could try restoring factory settings.