I have always used panniers in some 34,000 miles through nineteen countries, including going over the Rockies, the Alps, and many other lesser precipitous landforms; hills, hills, and more hills. I cannot say using panniers as opposed to a trailer will bring you some great advantage, or that a trailer is definitely better for such a tour than panniers, or for or against any combination of the two ways of carrying gear. I can say that panniers, whatever their advantages or disadvantages may be, are just fine for what you have mentioned. I can say many journals I have read on CGOAB attest to the suitability of using a trailer. Which method would be best? I do not think I could say. If I were going to cycle the route you have mentioned here, I would overhaul my touring bike, put on two front panniers, two rear panniers, and a handlebar bag, and take off. I would also mention that I would do it that way because I already have all the gear, and it would not make sense at this point to buy new gear for some theoretical advantage.
If you use a trailer and panniers, keep the panniers over the front wheel. I mean, if you use a trailer that can reduce the load over the dished rear wheel, what is gained by then hanging panniers over the rear wheel? It is the rear wheel that takes the load. I remember clearly wearing out three rear tires to one front tire. I do not remember ever having even one broken spoke in a front wheel. I remember replacing many spokes in rear wheels. If a trailer reduces the load on the rear wheel, keep it that way.