I don't think the snitch had to know that necessarily. If Doran or Hotah knew the area well enough and knew the final destination, they could rather easily have worked out which route Arianne would take to get there. Hotah got ahead of them, which means he not only knew how to get to the right spot, but possibly how to get there via another route so they wouldn't see him go past them (or he just got up insanely early, which makes sense, plus a lone rider can go faster than a group, don't mind me, just thinking out loud here). S
I still think it was Arys, but I'm okay with that being wrong.
sweetsunray, you've laid out a decent case but I'm not convinced that's what happened.
What about Tyene? If she and Arianne talked about a plan to crown Myrcella they might well have talked about different ways to go about it, potential routes to take, etc. And while Tyene was on house arrest she might have thought she could bargain for her freedom by telling Doran his daughter had a dangerous plan that could ruin everything. She could even have told Uncle Doran well in advance and he could have had Arianne watched a bit more closely, waiting to see if she actually would try it rather than condemning her based on Tyene's word alone.