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That is terrible! I would apply fungicides, or biofungicides to the soil just as a precaution. Flooding can cause major issues with infections, as roots remain suffocated in wet soil, so applying a fungicide as a preventative method can help keep plants from dying off.

Though there isn't much that you can do... this article may offer some more tips:

https://www.gardeningknowhow.com/plant-problems/environmental/flood-damage-garden.htm

I would amend your soil, then solarize it! This is an easy, carefree way to rid your area of all pests. Here are some articles that will help:

https://www.gardeningknowhow.com/special/organic/how-to-easily-improve-your-clay-soil.htm

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

https://www.gardeningknowhow.com/special/organic/the-importance-of-soil-for-an-organic-garden.htm

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

This article will help you to solarize your soil:

https://www.gardeningknowhow.com/garden-how-to/projects/how-to-solarize-garden-beds-to-eliminate-garden-pests-in-the-soil.htm

This is a complicated subject. If you seek to do this without much risk, then you are doing the best that you can. If you choose a chemical means to do this, you can be subject to highly carcinogenic chemicals, or compounds that have negative effects on wildlife, and will need to follow strict laws when applying these chemicals. Not all plant growth regulators, and sterilizer are created equal! Some are safer than others.

There may be people in your area that can do this for you, so you will want to contact your local extension for more information:

https://www.gardeningknowhow.com/extension-search

This article will give you more information on auxin inhibitors:

https://www.gardeningknowhow.com/ornamental/trees/tgen/fruit-tree-sterilization.htm

Yes, you can try that. If the remaining part looks healthy, you also can try propagating a cutting.

https://www.gardeningknowhow.com/garden-how-to/projects/rooting-plant-cuttings.htm

The object of fall planting is to plant 6 to 8 weeks before the first frost so the plant can get established. If you still haven't planted them, I would wait till spring now. Keep them in a cool environment such as a garage or unheated basement that is above freezing all winter. Water just enough to keep them alive through the winter, about once a month. Then after the last freeze, go ahead and plant them outside.

Sandstone is very porous so take care not to use any acidic cleaners as they will eat away the stone. Do not powerwash. The best thing to use is dish soap and warm water. First apply warm water and let it sit for several minutes to loosen dirt. Then gently brush dirt aside with a cane broom. Then with warm water and a few drops of dish soap, scrub the stone with a brush. For tough stains, use a paste of baking soda and water. Allow the paste to sit for several minutes before scrubbing. Once your sandstone is clean, apply a topical sealer.