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While these pests normally do not affect plum trees, this article may help: https://www.gardeningknowhow.com/plant-problems/pests/insects/treatment-bagworms.htm

I would actually suspect that a lack of water is the issue. Many parts of the country have experienced very hot and dry weather. Many plants that did not previously need watering during the summer now need to have supplemental water. With trees, when they become water stressed, they will literally suck every drop of water out of the ground around them, which makes it twice as difficult for plants to survive under them.

Try watering the tree. Make sure it is a good deep watering each time you do it. The tree has likely used up even the deepest reserves of water in the soil and it is killing both the tree and the grass.

Sounds like it may be peach borers. They can affect any stone fruit. This article will help:
https://www.gardeningknowhow.com/edible/fruits/peach/peach-tree-borer.htm

Yes, you can bolt the split together. Most of a tree's living tissue is at the surface of the trunk, so bolting the split together does relatively little damage to a tree and can help support it while it heals.

I would treat any wound areas with a fungicide to help prevent fungus and rot from setting and and keep a sharp eye out for pests and treat them quickly if you spot them. They tend to be attracted to weakened trees.

It depends on what the holes in the leaves look like as to what it may be.

If the holes look like they have been chewed out, it could be slugs. They have been known to get up in trees and eat holes into leaves at night - which is why you would not have seen them.

If the holes are ringed with yellow, it may be a fungus. In this case, treat the tree with a fungicide.

Most people are unaware that by putting paint, tar or other permanent substance on damaged trees, it can actually do more harm than good by trapping disease against the wounds.

I think the best thing you can do for the trees now is to make sure that they are getting plenty of water and perhaps even some fertilizer. Do not cover up any more of the wounds as this can harm the trees. The wounds need to heal in open air, but don't panic about the ones already covered. They should be fine. Just as a precaution, keep an eye on the tree for any signs of pests or disease around the damaged areas and treat the wounds with a fungicide like neem oil.