Would Santana (or Kurt, even) have kept a copy of the tape? I'm not especially versed in criminal law, so I don't even know if it would be admissible as evidence of tampering with the slushie, but we don't HAVE to follow canon. It's possible that they give the cops the tape and it still goes nowhere (maybe Blaine's dad refuses to press charges, since he knows it won't go anywhere, or maybe Sebastian weasels his way out of any consequences), but at least they'd have made the effort?
It'd need to be discussed with our Kurt and our Sebastian, but we can totally tweak the plotlines if it makes better sense...