That is quite a dilemma. I don't want to be the guy giving useless information that you have already looked into. Do you have an idea what the maximum load angle will be one you get your suspension and splitter?
A roll back will be quite an endeavor. I would still try to keep the rear axle as far forward as possible to give you a better pivot point. I have seen a few tilt decks that have the frame permanently attached to the deck and the tongue is what separates allowing it to tilt and pivot on the rear axle. Adding the roll back option to this seems like it could be the easiest with the lowest deck. Keeping load angles low. Personally I would keep the axles as far forward, while maintaining proper weight distribution, to also aid in the tilt angle. Also if you don't already have drop axles, I would look into those as long as it won't interfere with your GN overhang.
As a dream trailer, I always thought about replacing the leaf springs with air bags so I could dump the air for better loading.