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Black sooty mold is a fungi that latches on to the sweet, sticky honeydew secreted by many insects like aphids and scale. When the insects suck the sap, they excrete a thick gooey sugary substance called honeydew. Determine which insect is causing the problem so you can apply appropriate counter measures. Your local plant nursery can help identify them if you put them in a sealed, transparent plastic bag. If you find aphids or scale, treat your plant with Neem oil. Spray all parts of the plant extremely well but consider not spraying the flowers, which attract pollinator insects (the "good" guys). Follow the directions on the product's label.

I didn't find any publication that said to beware of massive roots, only that you needed space for a 6 ft. tall by 3 ft. wide plant. It is a slow grower so even that will take a while (most publications said 3 ft. by 3 ft. was the norm). I think you can plant it in the ground and be fine as long as you allow the 3 foot clearance.

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm

https://gardeningsolutions.ifas.ufl.edu/plants/ornamentals/crown-of-thorns.html

https://plants.ces.ncsu.edu/plants/euphorbia-milii/

They need four hours of sun a day, so I would move it where it can get more light. The article you referenced said crown of thorns was a forgiving plant, so hopefully it will look better in time. Follow its care instructions below.

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm

Usually a plant not flowering isn't getting enough light. Try moving it to a south or west facing window. You also can move these plants outdoors in the summer if you would like. Just be sure to bring it back in before the temperatures drop in the fall.

https://hort.extension.wisc.edu/articles/crown-of-thorns-euphorbia-milii/
https://plants.ces.ncsu.edu/plants/euphorbia-milii/
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm

Tucson is in USDA hardiness plant zone 9, so you are on the edge of its hardiness for outdoors (10 and higher). It can take temperatures and heat above 90 degrees F, so you can make a judgement call here. Zone 9 still gets some below freezing temperatures and crown of thorns cannot tolerate frost. If it's in a container you can always bring it indoors for the winter. Here is more:

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm

Yes, you can remove dead stalks at any time. Here is more:

https://www.gardeningknowhow.com/houseplants/crown-of-thorns/cutting-back-crown-of-thorns.htm