The coffins roll down the roller belt, but it's a non-powered belt. The coffins wouldn't move on their own, much less at the slow pace that they're seen moving. If Greta had shoved them, they'd be touching each other and would roll in quickly.
Uncle Lewis looks at the clock and says "Only 18 minutes until sunrise," but the clock reads 3:30AM. Sunrise on Halloween is much later than that -- closer to 8:00AM than 4:00AM.
When Uncle Lewis says there are 18 minutes until sunrise, the clock shows 3:30, which means sunrise would be at 3:48. A few minutes later, Jack says that sunrise is at 5:48.