I really don't know where all of these crack-pot "engineers" came from. I've never had a single problem with just the dirt I took out to put the post in. But, my oldest wood fence is only 30-years. Concrete is a total fraud invented by idiots, thankfully you got the right advice there already.
- I don't have a big problem with Tamped gravel, except there's no purpose in an average soil situation. No, gravel doesn't "wick" anything away, that's very wrong advice. Gravel just drains into the surrounding soil, therefore no purpose...unless you're removing the soil everywhere.
- Just go down (a minimum) of half the height of the fence or 1/3 of your post. 6' post goes 2', 9' post goes 3'... again, minimum. If you think the load is possibly excessive, then you'd use 9' posts at 4' deep for a 4' or 5' tall fence. You might even consider 4x6 or 6x6 posts...excellent for rail fences.