Maybe they do, maybe not. McG has history with quidditch which makes her love it enough that any perceived injustice in it would bleed over into her personal relationships. The other professors really aren't shown loving it enough that the bleeding in would happen. If they worry about the points, surely they have more reason to dislike Snape, who mostly takes points from other houses, not his own
400 were fighting against the order, assuming you're getting that number from his offhand comment of 20x order members. That doesn't mean all of them would be death eaters. Death eaters seem to be much limited in number. Not everyone is given the mark.
When voldemort returns, he names two that have been lost. One is karkaroff, the other Snape. The rest are either in prison or have come at their master's call. He may have lost more supporters, but in terms of death eaters, he's only missing two. Unless the ministry somehow got 400 DEs either captured or killed. That doesn't seem feasible. When the azkaban is broken, only 10 or so death eaters come out. That hints toward a lower number of marked death eaters. The rest of the ranks would be filled out by snatchers and creatures. They would still be counted as fighting the order, which is what Remus was likely referring to
Skeeter covered the next trial. The ludo bagman one. Still, it's hardly feasible that a named death eater would not make waves, especially when he's vouched for by albus dd, who confirms infront of 200 witches and wizards at Igor's hearing that Snape was a DE but had defected.
The wizarding community is 10k strong at the maximum. They have one school. A school whose headmaster just admitted that one of his staff members was a death eater. It's impossible for this not to get back to the staff. It's simply a numbers game. There were no measures to ensure silence. Why wouldn't they tell their child to be wary of the death eater teacher? Why would news from these 200 witches and wizards not reach the staff who stand at the centre of the wizarding world?
Sirius doesn't know anything because he went to azkaban before the trial happened. His main objection is that Dumbledore wouldn't hire someone who was a death eater on staff (which should be even more telling of how much it would be gossiped about among the 200). It's his belief in DD, not his belief in Snape, who he knew loved the Dark arts, that makes him say that to reassure Harry.
Fudge is an outsider, for all he is minister. Snape is not someone who'd be very interesting to outsiders, but very interesting for people with children and the teachers of Hogwarts. Maybe he forgot, maybe he never really thought of DEs and pretended to forget that one of his backers was one, maybe he knew, and that surprise is due to seeing the mark for the first time, so black, rather than Severus being one.