Gravel and Cement Use for Chain Link Fence
The point of the gravel is not just for rot prevention.  You want to dig down below the frost line, first and foremost, so that any water below the post won't freeze and heave the post out of position.

The gravel below the post allows water a place to pool until it eventually drains away completely.  If you put the concrete directly in the ground then water could gather around the concrete at or above the frost level and cause heaving to the side.  Having that extra drainage helps prevent that.

So in general it's a good idea to put gravel in ALL your holes.  Critical?  Not necessarily.

Your last question - kinda hard to visualize and definitely on a different topic from the first two. You may want to split that part out to a new question.