Many of the above could be seen as correct.
Voldemort would arguably know that McGonagal and Flitwick were part of the Order, just as much as he'd know Hagrid was. What he wouldn't know is which of the students were. The Professors could be seen as trophies but also as symbols that there is no resistance.
He'd be less concerned with the education or the parents than actually controlling these young, impetious wizards. Despite his mocking of Neville, there is some admiration there for his "pluck" and he'd be a valuable ally if he could be turned for example. Prior to the battle, the best way to keep the students docile and compliant is to present as close to "business as usual" as possible rather than cracking down by killing the old guard in front of them. He is bargaining when he is asking about Harry rather than outright threatening, he is telling them "your lives won't be much different except you now serve me, but if you don't your lives will be over." The threat is always present but he rarely overtly threatens people who he thinks might have value to him, he offers them the chance to join of their own free will...
He knows those 2 teachers and particularly McGonagal would never risk a students safety for herself whatever the personal cost to her own moral and likely hopes she too will eventually join him willingly and at minimum will do as told without the need for the Imperius curse at pain of killing off kids as punishment. He'd never kill her, just make her watch him kill her Gryffindors one by one.
Hagrid is useful from the practical point of view. He performs functions at Hogwarts that need performing whoever is in charge, again he won't risk the kids safety by being surly or rebelling outright is Voldemort's thinking, he knows Hagrid's weaknesses and can play on them easily.