"The trail curled higher" here means that the trail was going upward, but in a curved path, not a straight line. I think the second passage means: "The trail sloped downward into the old church, and Ellen slipped slightly" (from the uneven floor) The third passage means that the dogs were smelling ("snuffling" means sniffing loudly) at the bottom of the stairs. The dogs were walking from side to side ("crisscrossing"), and the fur on the dogs' backs was standing up (a sign that the dogs feel threatened or something like this.)