Will these spikes stay on during driving or do they fall off?

The spikes are designed to be user-removable for aesthetics—not quick-detach from road vibration. Once installed hand-tight, they stay put until you twist them off intentionally.

Do these come with any mounting hardware or washers?

Set includes the 20 lug nuts only. No washers or additional hardware are included—you're working with what's already on your wheels and hubs.

How do I know if M12 x 1.5 is the right size for my vehicle?

Check your vehicle's service manual or the markings on your current lug nuts. If your wheels are currently fitted with M12 x 1.5 bolts, these will thread directly. Mixing thread pitches will result in poor seating and safety risk.