Can alleged nerve agents ‘survive’ that long in the environment (assuming that the two incidents are related)?
Yes, easily, especially if protected from water, air or sunlight which would be the main things that might degrade them,
Some are designed to be ‘persistent’ so that the risk of poisoning continues and anyone working in the area has to take precautions, which degrades performance and causes casualties. You might use those on an enemy’s airfield so they’d have to try and run air operations while all suited up. Others are non-persistent and degrade quite quickly. These would be used, for example, if you expected your own troops to occupy the area soon and didn’t want to expose them to risk.
The Litvinenko poisining left radioactivity in many sites around London and a few further afield, and where the poisoners prepared the polonium was hot as Hell. No reason to expect the users of the Novichok would be any less messy.