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Since it is a large tree and fairly old, it may not be practical to fix the leaning. That being said, if you would like to give it a try anyway, tie a rope around the tree trunk and anchor it in the direction you wish teh tree to go. Use a piece of garden hose over the rope where it touches the tree to protect the plant. Tighten the rope a little every few weeks to pull it back into the right angle. This will correct it without hurting the tree.

Maples are usually best pruned in late summer/early fall, though may be done in early spring when necessary.

It is a toadstool. They do not pose any threat to your tree, but if you want to get rid of it, you can just treat it with a fungicide. This should take care of it.

There is a product called Preen that keeps seeds from germinating. It comes in both a chemical and a organic version. Or there is corn gluten, but Preen is easier to find. You would just spread it over the area and this would stop all seeds in that are from germinating.

How often are you fertilizing and how much are you giving them when you do? You may be overfertilizing, which can burn the roots and kill the plants.

There are many reasons this can happen to a tree and it is extremely difficult to diagnose without being able to examine the tree. As you have lost one maple, and are losing another, I would say that it is likely a maple specific disease, but beyond this I can only recommend having a tree professional come and examine the tree to see if they could determine the cause.

Unfortunately it is a bug infestation. The sap is actually honeydew, which several different pests secrete, such as aphids or scale. I would treat the plant with neem oil. Neem oil is very effective on these kinds of pests and is safe for people and pests. This article may help:
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm