The cambium layer is just under the bark.  The cambium layer  contains vessels know as the phloem that transport sugars, generated through photosynthesis in the leaves, to the roots. Once these vessels are damaged and transport of sugars  is no longer possible.  The large amount of missing bark on the middle trunk has disrupted the fluid movement within the second trunk.  The branches in the middle canopy may soon die due to the the loss of phloem, xylem, and protective bark. 
Cankers on mulberry tree twigs, branches, and die back of twigs may be caused by at least six fungi: Cytospora sp., Dothiorella sp., Gibberella baccata, F. moricola, Nectria sp., and Stemphylium sp. These can be distinguished only by microscopic or laboratory tests. Prune and burn dead branches. Keep trees in good vigor by watering and fertilizing.
Thick, black, shoestring-like, fungal strands  grow in a net on  armillaria infected trees and in the soil around the base of the tree. Clusters of honey-colored mushrooms grow at the base of the tree in fall. I do not see the fungal strands on the middle trunk. It may be too early in the season for the mushrooms.
https://extension.umn.edu/plant-diseases/armillaria-root-rot
Have this tree assessed by a certified arborist to determine the structural stability. https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional      In conclusion, the second trunk has serious problems. As it grows it is pressing against the bark of the other two trunks.  A certified arborist can examine this trunk and determine the risks for falling in a storm.

I did not find any research stating whether the berries were safe to eat once a tree is infected. The berries were not considered in any of the papers that I read.

I am sorry that your tree has this problem.