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You can use things such as copper antifungal solutions, or you can use wettable sulfur. Either of these will take care of the issue, as long as it isn't one of the "incurable" species of fungus. I prefer wettable sulfur, as it provides things that the tree will likely need anyways.

You might want to add dolomitic lime as well to keep the pH in the soil a little more stable while this is working on the soil.

Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/apples/apple-collar-rot.htm

Wow... I have never seen that before, and my research doesn't show much on the subject either! I'm willing to bet that it is something, either, bacteria or fungal in nature. You can try adding wettable sulfur to the area. This can help with the situation but it may not sure it fully.

I would suggest taking a small sample to your local extension and see if that can help you identify the issue.

This article will help you find your local extension: https://www.gardeningknowhow.com/extension-search/

Those are tiny plants called lichens. In general they are harmless and common on fruit trees.

Pruning generally takes place in March and April, so you are right on schedule.
Your soil may be an issue if it is not well draining.
It can be more difficult to deal with this when you have all ready planted.

I have listed several links for you to help you get started with care of your new trees.

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/what-is-well-drained-garden-soil.htm

https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-planting.htm
https://www.gardeningknowhow.com/special/urban/growing-columnar-fruit-trees.htm
https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-feeding.htm
https://www.gardeningknowhow.com/edible/fruits/apples/pruning-apple-trees.htm

This depends on the amount of damage on the trunk. If 50 percent or more of the trunk is damaged, the tree may become girdled and die.

Less then 50 percent damage, you can try some good old TLC.
Mulch the ground with 3 to 4 inches of hardwood organic mulch. Keep it at least 6 inches away from the base of the tree.
Provide moisture if rainfall is sparse.
Protect the tree from future damage by using hardware cloth to make a barrier around the tree.

Here are some links with more information.

https://www.gardeningknowhow.com/garden-how-to/mulch/best-mulch-for-garden.htm
https://www.gardeningknowhow.com/ornamental/trees/tgen/rabbit-damage-to-trees.htm
https://www.gardeningknowhow.com/edible/fruits/apples/apple-tree-planting.htm

What, exactly, happens to them? If you can find a picture of the issue or describe it to me, then I will be able to help you better. It could be so many things, and some of them will not be treatable, while others are.