Since you´re going on a cruise I assume you have a cabin. Bring it all and adjust to whatever situation/opportunity that comes your way. The rest you can leave in your cabin. I know I would have had exactly the same issues as you and ended up with breaking my back getting it all onboard the boat.

I have pretty much the same setup as you, apart from the 300. I have solved that with the 70-200 and the 2x or 1.4x extender (not the same IQ though). I have just bought the 600/4 IS II. I have used the 400/2.8 IS II up until now, but since both the 5DIII and 1DX can AF at f8, I went for the extra reach. It´s coming tomorrow

I fully understand your thoughts about the 85. In my view that lens produces some of the most stunning 3D pop-out portraits, with the most beautiful bokeh you can get from any lens at any price. Very shallow DOF at 1.2 though, so it can be frustrating now and then, to have portraits with sharp nose tip and blurry eyes.

Lens I would carry that you left out is the 24-70/2.8 II and the 100 2.8 Macro. The 24-70 is my walk-around lens. It produces stunning results. I like the macro lens a lot and in Alaska you should find lots of good use for it. I would definitely bring both bodies. They are both great bodies, but they have slightly different qualities and you definitely need a back up. I normally skip the extra grip for the 5DIII tough. When using the 600 you will most likely want to have one of the other lenses on the other body.

Enjoy your trip!
Eldar