One thought is to make a list from a specific import and then query the list. But I don’t see a direct approach in the Search API documentation to accomplish something similar. Not by specific import. We could look at createdate, but that could pull in contact created that day that are not part of the import.
Q2 – I do not have any documentation showing fileIds as an externally useable property or value. In other similar cases, I’ve seen values like this get retuned via a public endpoint, but only have internal (HubSpot) uses. Confusing, but it happens.
For Q2, can you tell us more about how you are using this value or were thinking of using it? I am happy to dig a bit more once I understand the context more.