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The following article should be of some help to you: https://www.gardeningknowhow.com/plant-problems/pests/insects/prevent-caterpillars.htm

Prevention is key with regular checks of the undersides of leaves, especially during the spring and summer, which may turn up moth or butterfly eggs that will develop into caterpillars. Simply pick off the eggs and dispose of them. For a smaller number of caterpillars, simply pick them off, relocating them elsewhere or squash them. They can also be dropped into a small bucket of water and drowned. Beneficial predator insects are another option for control. Some quick and easy homemade repellents to keep caterpillars from eating your plants include solutions of water with one of the following plants added: onion, garlic, chives, hot pepper (jalapeno, cayenne), or an herb (basil, coriander, wormwood, peppermint). Steep the leaves in hot water or mix in a blender and strain before spraying. These sprays will successfully repel a wide range of insects. Flour and salt can also be used as a dust to suffocate or dehydrate many types of caterpillars.

The brown structures are gone to seed flowers, prune down to the nearest node and you will get a second flush of flowers on each pruned stem, they will however be smaller than the first flush.

Yes, it is a process called creating a standard topiary. It is done often with hibiscus, which have a similar growing patten as butterfly bushes. This article will help: http://www.hgtv.com/landscaping/the-makings-of-a-topiary/index.html

Yes, the seeds can be saved for making additional plants, though this will take much longer, so cuttings are generally recommended. This article will help: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/propagating-butterfly-bush.htm

This article will help you with that: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-pruning-how-to-prune-a-butterfly-bush.htm