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This sounds like it could be a mild fungal infection. I would suggest using a fungicide on the upper portions of the shrub. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

I would also treat the soil with a handful of DOLOMITIC LIME and a teaspoon of WETTABLE SULFUR once or twice each year. This will help to prevent the issue in the future.

This article will help you to grow Hibiscus: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm

This is not a cross pollination issue. A cross pollination issue will only show in the next generation seeds.

I'm afraid that this is caused, partly, by genetics and also by environment. This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-turning-different-color.htm

Most miracle grow products contain slow release nitrogen, and quite a bit of quick release nitrogen. This will halt flowering. This would be my first guess. You will have to offset this with potassium and phosphorus, or by raising the pH just slightly.

This article will give you more information: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-fertilizing.htm

This article will help you to grow Hibiscus properly: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm

Cutting them back anytime outside of early spring will result in less blooms. It is important to only cut them back in early spring except for tip pruning.

This article will help: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm

It will depend on the type of hibiscus that you have. Some are cold hardy, and some are not.

This article will help you to overwinter them: https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/wintering-hibiscus-indoors-winter-care-for-hibiscus.htm

You can do light pruning and tip pruning during the summer but the serious pruning should be done in spring.

This article explains:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/tips-for-pruning-hibiscus-plants-when-to-prune-hibiscus.htm