My situationship suddenly went cold about going to prom together, and I found out my best friend has been privately feeding her rumors about me that aren't true. How do I address this without completely blowing up our entire friend group two weeks before prom?

Address it with her first, not him. The fastest way to disarm sabotage is to cut off its audience before you confront the source. Friends who quietly poison your romantic leads usually do it out of jealousy, fear of being left behind, or a hidden interest in the same person. If you confront him first, he’ll likely deny it, warn her, and make you look paranoid or aggressive to the rest of the group. Ask her to talk in person and say: "Things felt different between us lately, and I heard some weird rumors might have reached you. I really like what we have going, and I’d rather you hear the truth directly from me." Clear the air without attacking him yet. Once that's settled, take your friend aside privately. Keep it short: "I know what you said to her. Why did you do that?" Then just stay silent and let him sit in the discomfort of having to explain himself. Real friends don't run covert operations against your happiness.