1) For landscape, you'll want a Crown Graphic. The focal plane shutter and larger body of the Speed makes it much heavier (though Speeds are great cameras too for focal plane rangefinder uses, i.e., portraits or street, for example).
2) I use a nearly naked Crown Graphic for landscape work. It has the top rangefinder and internal focusing components removed. This reduces the weight to about 4.5 lbs without a lens. You can easily remove either kind of rangefinder.
3) On a Crown, I use my 65 4.5 Grandagon on the inner track with the bed dropped; it works beautifully. The Crown has room to go wider.
4) A Crown Graphic will take something like a Horseman 612 back, yes.
5) Crown Graphic.