You do not ask EASY questions!
(1) Do we need to level out that 2 foot lateral (side to side) drop? If we do not, I am thinking the 172will want to wander down that lateral slope, making it impossible to back the 172 into that space.
Since you are placing gravel in the area, it seems to me that you would be OK. I would be more concerned in loose dirt or mud, regarding sliding/wondering. If it were me, I would take dirt from the "high" side and throw it to the "low" side. Then gravel on top.
(2) Can we back the 172 down the 2 foot slope over its 6 foot run (toward the photographer) to the level area? Or do we need to level the entire area? We are loathe to do that because that means we would have a U shaped retaining wall 2 feet above the surrounding yards.
Too tough (for me) to call while sitting at the desk. I would have a driver and a "spotter". The spotter would closely watch (from a safe position) the underside of the trailer for impending problems, as the driver slowly backs up. The transitions from level to slope can cause things to "scrape" underneath.
Gravel is not the best traction surface so, take it slow and easy. If your vehicle has 4WD, I'd use it.