Wow, that was a lot of pages of posts to read. My OTTB goes great on trails when he is with other horses. When he is not he does the snorting and looking around OP mentioned. I've found that miles work best for us. He's always a little hot out of the gate but he calms down the farther we go. He does do the "stop and drop" now and then but it's mostly like a big flinch. Then he moves on. He's never done anything to endanger me and always seems aware of my presence. So I say, if arena works makes you feel better about going out on trails, go for it. But for Comic and I (he hates the arena) the more wet saddle blankets we make the better we both do. It seems like OP and her horse have the relationship already, I'd start logging some miles.
Riding: The art of keeping a horse between you and the ground. ~Author Unknown