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It sounds like a pollination issue. This article should help: https://www.gardeningknowhow.com/edible/fruits/plum/plum-trees-not-fruiting.htm

It sounds like you have a fungus. Disturbing the roots probably did not have a direct effect, but it may have weakened the tree a little, which made it more susceptible to fungal infestation. Treat the tree with a fungicide and this should clear up the leaf drop and the black leaves.

This article will help with pruning: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm

This article should help: https://www.gardeningknowhow.com/edible/fruits/plum/plum-tree-pruning.htm

It is just lichen on the trunks. It is harmless. You see it very frequently on trees that are sick because they tend to be attracted to unhealthy trees, but they also grow on healthy trees so their presence does not indicate that the tree is sick. This article will explain more:
https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm

Leaf drop is one of the ways a tree deals with stress. When a tree is not getting enough water or nutrients, it goes into emergency mode. First it will shed its fruit, then its leaves in an attempt to reserve its resources to try to live.

I would make sure the borer problem is not more extensive than you currently think and also check the roots for signs of damage. Give the tree some balanced fertilizer as well.