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Have you seen the "pests"? It could be due to the wonky weather instead. Most people experienced a fairly mild winter, resulting in early budding of many trees and plants. This was followed by sudden cold spells, which could have possibly caused damage to these buds. This too would cause the symptoms you are seeing.

Yes, if it has gotten unsightly, you can prune it to your desired appearance. Just take no more than a third of the plant at one time.

Hose down the plants and then treat them with neem oil. It is both safe and effective against these pests. Here is more information: https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

That could be possible, but only if the wall is relatively new, say just a few years old. If it is older than that, the chemicals in the concrete would have leached out long ago and be gone.

There are many things that can cause yellow leaves on a plant. This article should help you get some ideas on what could be affecting your tree:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

I think the culprit here is scale, though mealy bugs is naother possibility. It also sounds like the tree has sooty mold (the black you are seeing), which is actually an indicator of the pest infestation. Many times these pests are too small to see, but they can be damaging. The stickiness you notice is honeydew, which they secrete. I would recommend treating the tree with neem oil. This will treat both the pests and the sooty mold fungus. Here is more information that may help: https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Oftentimes, galls can be spawned by insect pests, like aphids. If you are seeing ants, this would be a clue that aphids (or similar sap-sucking insect) are present, as they are attracted to the sticky residue these pests leave behind (honeydew). You can treat the tree with neem oil to get rid of the aphids, which in turn will get rid of the ants as well.