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Unfortunately you will need to wait and see.

The duration of the flooding causes the roots to suffocate.
Here are some links with more information.

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

https://www.gardeningknowhow.com/ornamental/shrubs/ligustrum/growing-ligustrum-shrubs.htm

This sounds very much like cerospora fungus. It can be due to damp weather or excess moisture levels. If your hedge is not receiving good sunlight also can cause issues.
Prune out the diseased material and dispose of, rake the ground and clean up the litter.

Treat the plant with a fungicide like myclobutanil or Immunox.

https://www.gardeningknowhow.com/ornamental/shrubs/ligustrum/growing-ligustrum-shrubs.htm

You will need to dig up and remove the root ball.

https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-kill-a-tree-killing-trees-in-your-garden.htm

Review the fertilizer schedule in the link below and compare with the applications you made to your shrub.
You can try flushing the area and pruning out the damaged plant material.

https://www.gardeningknowhow.com/ornamental/shrubs/ligustrum/growing-ligustrum-shrubs.htm

This seems to be a fungal infection. Very strange though, as it seems to force it to flower and die at the site. There are a few ways to help cure the area of the issue, but I recommend using wettable sulfur, along with some dolomitic lime to buffer the soil. This should help clear up the issue.

Yes, these tend to be very vigorous, and will do just fine with transplant. I wouldn't wait much longer though. Once it gets hot, there is a much greater chance of death of a plant.

This article will help you in transplanting shrubs and trees: https://www.gardeningknowhow.com/ornamental/trees/tgen/transplanting-trees.htm