CDC updated their quarantine guidance, which now states 7 days after travel, if one tests 3-5 days after travel. I'm beginning to wonder if MTV tested them after travel at all, just tested them before, quarantined for 7 days and then let them loose. In which case, they could very easily miss a positive test.
And yeah, I would guess the entire cast and crew now has to quarantine for at least 7 days (with a negative test on day 5 or later) or 10 days without testing. But like you alluded to, if anybody else develops symptoms, that's game over for the season, I'd imagine.