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With any oil based sprays, it is very important to follow all packaging instructions very carefully. Oil sprays are not meant to be used in hot weather, or in bright light. These conditions will cause the oils to burn the leaves. It is also very important to, only, test on a very small portion of the plant to make sure that it does not have a reaction to the oil.

I would try and force growth from the top, first. As long as the stems are not brittle they will likely leaf back out. If the top will not leaf back out, then you can allow any growth from the bottom to take back over.

Here are some articles that will help:

https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm

https://www.gardeningknowhow.com/special/organic/what-are-organic-pesticides.htm

https:/c/www.gardeningknowhow.com/special/organic/natural-home-pesticides-organic-garden-pest-control.htm

Depending on the condition of the slate, how much of it, and how dusty it was, then there is a likelihood of the soil pH having been raised too high. This can be confirmed with a soil test, and pH test. These articles will help:

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm

https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm

This article will give you more information on the care requirements of the plant:

https://www.gardeningknowhow.com/ornamental/foliage/new-zealand-flax/new-zealand-flax-plant-care.htm

It is not unusual for the plant to revert back to its parentage foliage. If this happens, cut the affected leaves back to the base of the plant.

https://www.gardeningknowhow.com/ornamental/foliage/new-zealand-flax/new-zealand-flax-plant-care.htm

http://sonomamg.ucanr.edu/Plant_of_the_Month/Phormium/