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Newer cultivators have larger leaves then older varieties.

Crown of Thorns are susceptible to fungus issues. You can use a fungicide on the plants.
Bacterial leaf spot can also be found on these plants. The disease is characterized by circular gray to brown leaf
spots surrounded by distinct yellow halos. The infected leaves often turn
yellow and fall off the plants. Leafs should be removed from the plant and disposed of. Make sure the plants have adequate air circulation.
Avoid getting water on the leaves and stems.

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

This looks like Mealybug.
Treat with Neem Oil.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm
https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm

This appears to be something of a fungal issue, and more than likely a pH issue, also. Apply wettable sulphur to the soil, to control the fungus. Add dolomitic lime to the soil to correct the pH. Here is an article for more information on the plant: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/growing-crown-of-thorns-outdoors.htm

This sounds like a fungal issue. This can be caused by the soil remaining wet for too long. Usually, this can be corrected with something as simple as wettable sulfur. This will take care of the infection rather quickly. The old growth will not recover, but new growth will come out unaffected. Just be careful when cutting off any dead growth, as these plants can cause some painful irritation to some.

Here is an article for the care of these plants: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm

The only thing you can do is let those leaves fall off, and bring it in to recover on its own. If ,after a few months, it starts to put out growth again, you may give it some fertilizer to help it come back. You don't want to do this until you know it is on its way to recovery, though.

This article will refresh you on the care needs of this plant: https://www.gardeningknowhow.com/houseplants/crown-of-thorns/crown-of-thorns-houseplant.htm

Yes, you can prune a crown of thorns by removing up to 1/2 of the plant's total size at once. You can prune individual branches back, optimally to where a lateral branch emerges or where you see a bud so that the plant will regrow from that point. You can also remove whole braches if the plant is too dense. Wear gloves while pruning your crown of thorns, because it contains irritating, toxic sap.