Ok, mincing words, yes, the jury can decide pretty much anything, whether relevant or not relevant. Yes, in a murder trial, they can come back with a verdict that hot-air balloons rise. Then the Judge can step in to fix, even to the extent of vacating it for a new trial.
Yes, I know that a judge MAY fix things. He CAN fix things. But he CAN and MAY also FAIL to fix things. Our legal system has NOTHING NOTHING NOTHING NOTHING to do with law. Roe v. Wade? How about 1 trillion unconstitutional gun laws and rulings on every jurisdictional level in this country? Fed judges overturning gay marriage laws, voter ID laws, and a trillion other legally passed laws?