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This seems like a fungal infection. Since this is in container, it will be easy to treat. Mix 1/4 peroxide with 3/4 water and water in once. Make sure to let the soil dry out completely down to about 2 or 3 inches down before watering again. This will help prevent infections in the future.

You will also want to try and remove any infected portions of the plant if you can. This will help in getting rid of the infection.

This appears to be a type of Yucca! This article will help: https://www.gardeningknowhow.com/ornamental/foliage/yucca/caring-for-and-landscaping-with-yuccas-outdoors.htm

Yes, but only mildly. It will cause upset stomach, vomiting, and diarrhea. It would be best to keep it where they cannot get to it. They usually won't eat it unless they have no other food source, though.

It certainly could be! Please include some photos and I will be more than glad to help you further.

Saponins are responsible for the wetting effect. These in combination with neem oil should not have much of an impact on the soil, besides killing fungus gnats, if that is your issue here.

As long you you have the proper buffers in your soil, such as dolomitic lime, then you won't have to worry much about your pH, as it will stay close to 7.

Now, if the problem is fungus gnats, then they don't cause much damage. In fact, they eat fungus! They only show up outside of the soil when it is too wet for them to breathe, and they consume fungus that may otherwise be harmful to your plant.

It would be a good idea to clip off anything that is completely dead. It can also help to fertilize with an all purpose. This article will help you with the care of these: https://www.gardeningknowhow.com/ornamental/foliage/yucca/caring-for-and-landscaping-with-yuccas-outdoors.htm