It is not possible. Blue and I have this problem every identathon–we’d really love to be able to exactly count how many unknowns were identified, but you can’t simply because once they are identified, they’re not unknowns. API calls ask for the current status of observations, not past status. And also there’s no such filter for “number of IDs on an observation=1” or “first identifier is not the observer.”
That being said, absolutely you should ID unknown plants to kingdom. I filter for kingdom Plantae every day and I have multiple helpers who put the plants in there from the unknown pile. At least one of those people just puts “Plantae”, and yet the two of us together have identified 2,019 observations, of which 1,217 are currently at species.