The way the system is now, if you gather evidence by "torture" (remembering it's just an analogy) and fail to come up with a convincing cover story for how you gathered the evidence, then the case is dismissed and everyone goes home. "Welp, it was worth a try!" It's this current system that's at risk of legitimizing torture as yet another illegal-but-oh-well tool for investigation.
Not if the victim of torture can afford our legal system. A (most likely Federal) judge would deny qualified immunity to those responsible, and they'd be held accountable at least in civil court, possibly criminal, although doubtful, given the way state prosecutors crawl up the police butt.
A federal judge deny qualified immunity? I'm struggling to contain my incredulity. There are instances where cops shot people who were unarmed and attempting to comply, where filmed doing it from multiple angles, and the cops still walked.
Federal courts do this routinely on cases brought before them. But most victims can't afford the court system. Here is a recent, typical one:
"New Brunswick, N.J., police drive their patrol car into fleeing suspect. Officers: We parked in his path, and he rode his bike right into us. Third Circuit: Video inconclusive. No qualified immunity.