Re: fence post footings
IMHO, you'd be better off to dry pack the post with dirt, as gravel will create a "dry well" for water to collect. Also, pea gravel does not compact, so you'd end with a very unstable post.
Post longevity has more to do with the general moisture content of the soil. If it's always wet, regardless of how the post is set, it's going to rot. If it's always dry, or at least can dry out over time, then the post will last for a very long time.
My recommendation is to not use wood at all. Instead, use metal posts such as the Post Master brand of post. Yes, the initial investment is a little more, but it will be a stronger post that will outlive you. This would be particularly important in a wet location.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!