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You can trim the seed heads, if you would like. Otherwise, there isn't much to do when caring for these shrubs. Here is an article that will help:

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

Unfortunately, none of the photos came through. I am unable to see the damage. Without treatment, bacterial infection from overwatering will carry over to the new soil.

It sounds like multiple infections. It would be hard to say which without seeing the damage, though. Fungicides will take care of the issue. This article will help:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will help you to care for Plumbago, properly:

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

I suggest cutting off the seed pods as they will stop the blooms from coming.

Plumbagos are drought-tolerant plants, and they do not need watering more than twice a week during the warmer summer months. But, that is once they are established after a few years and not if your soil is sandy and drains too well. When the weather cools and fall begins, reduce watering to once per week. I suspect they may have gotten much bigger and need more water than before and with the recent heat waves, they are not getting sufficient soil moisture. Consider adding some mulch to help minimize soil moisture loss. Then ensure you water thoroughly until all the soil around the plants is moist. Water again when the soil feels dry or almost dry. Too much water logged soil can promote root issues. Browned leaves due to watering issues may take a while to leaf out during the hot summer months. Blue plumbago is best grown in slightly acidic soils that drain properly and have a light texture. If your soil is alkaline, consider amending it with some manganese sulfate. More info: https://www.gardeningknowhow.com/ornamental/shrubs/plumbago/growing-plumbago-plants.htm A soil assay of your garden soil may be useful and indicate nutrient deficiencies or toxicities.

You probably need to overwinter it in a container. You are on the edge of its hardiness zones of 9-11. You can bring it indoors and place it near a sunny window for the winter. Don't water as much as you do outside. And don't fertilize during the winter. Here is more on its care:

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

Unfortunately, your photos did not go through, so I am unable to give a good diagnosis. They are relatively disease resistant, which leads me to believe that it may have something to do with nutrient levels or moisture levels. I would suggest testing your soil, and making sure to water after the soil dries several inches on the top.

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

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

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