Should I cut the branch off? If so, how do I dispose of it? Do I treat the “wound” where I cut it off? Most of the branch broke off in the Spring in a heavy wind storm
You can remove the branch with the conks, but you don't need to seal it. Make a clean cut and let nature heal it. You can cut it into small pieces that will fit in the trash.
https://www.gardeningknowhow.com/ornamental/trees/tgen/heart-rot-in-trees.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/ganoderma-root-rot-disease.htm
https://www.gardeningknowhow.com/ornamental/fungus-lichen/tree-bracket-fungus.htm
It appears that a buck sharpening his antlers may have been the cause of bark having been stripped from my crabapple tree. How do I treat the damage? How do I protect the tree from deer in the future?
Yes, it looks like deer damage. Here is what you can do:
https://www.gardeningknowhow.com/plant-problems/pests/deer/deer-rubbing-tree-bark.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/repairing-tree-bark-damage.htm