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These are best grown in cactus-type soils. These can be split, but it is best to wait until any other pups are rooted themselves before you split them. Otherwise I would leave them alone. If you have separate root systems, then they can be split similar to aloe. Here is an article for more information on their care: https://www.gardeningknowhow.com/houseplants/zz-plant/caring-for-zz-plant.htm

This are fungus gnats. These come out of the soil when it remains too moist for too long. They are a natural part of the soil ecology, and help to eat any rotting material in the soil, which will keep the plant from getting infections. If there are so many that they are swarming, it could mean that the soil has been moist for long enough for roots to start rotting. I recommend only watering once the top two inches of soil are dry.

If you still want to get rid of them, then you may put several drops of rosemary oil in some water for the next time you water the plant. You may also want to put a teaspoon of peroxide with it to make sure that any infection gets taken care of since there won't be as many organisms to eat any rotting material.

Here is an article for more information on the care of these plants: https://www.gardeningknowhow.com/houseplants/zz-plant/caring-for-zz-plant.htm

Can you include a photo? This can help me to see what is going on here. Are there any other symptoms, or sap on the plant?

Let the soil dry out and then cut away any parts of the plant that dies off.

Water less and make sure the soil is draining.

https://www.gardeningknowhow.com/houseplants/zz-plant/caring-for-zz-plant.htm

Yes, but rarely, so congratulations!

The plants will flower and the time for flowering is usually in mid summer to early autumn. The flowers a small and tend to vary in color from bright yellow to brown or bronze. The flowers are usually hidden among the bases of the leaves and are usually not considered particularly attractive. They can be removed without damaging the plant.

Allow the soil to become dry at the top to the touch between watering and do not over water. It's best to water this plant less than too much because over-watering can cause stem and rhizome rot.
It appears the plant is too deeply planted into the pot. Leaves should be above the soil surface. You can repot and place it correctly in the pot. Make the pot has an adequate drainage hole to allow excess water to drain.
https://www.gardeningknowhow.com/houseplants/zz-plant/propagating-zz-plants.htm