Premade reports have no more power against you than 1 report, and they will still have consequences for harassment and false reporting.
Can they throw the game as a group? Sure. Is this common? No, it isn't. There's no reason to pretend they are doing this any more than they would alone; they will be held accountable for their actions whether they are grouped or alone.
You seem to have somehow created this idea that premades are a hivemind of toxicity, as if one of them being toxic or wanting to throw the match means they all do. They are individual players, and grouping doesn't suddenly make them a problem.