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There could be several reasons for leaf drop. The container may be too small for the root system at this point; the bush may have depleted soil nutrients; watering infrequently or not deeply will stress the plant or it may be heat stressed. Large plants in containers don't, in my opinion, do as well late in the season as in-ground plants. Soilless mixes heat up, dry out (and are hard to remoisten) and have few nutrients late in the season. Any one of those conditions causes stress to the plant. Now would be a good time to cut the bush back and transplant it into the ground for the winter. Only fertilize with compost at this point in the growing season.

Although they typically don't do a significant amount of damage, I could certainly understand why you want them gone. I recommend neem oil, rosemary oil, mint oil, or any other naturally strong smelling oil.

Here is an article for more information: https://www.gardeningknowhow.com/plant-problems/pests/insects/controlling-boxelder-bugs.htm

It's completely up to you! This bush responds well to any type of pruning at anytime. Take them down to whatever is necessary for your case. Here is an article that will give you some information: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-pruning-how-to-prune-a-butterfly-bush.htm

In Georgia, butterfly bushes are best transplanted in the fall. This article explains the steps you'll need to take:
https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/tips-for-transplanting-a-butterfly-bush.htm

Where I live, the bush dies back in winter almost to the ground. While it doesn't have to pruned if there isn't winter die-back, it flourishes with hard pruning. This is because it blooms on new wood. Pruning also helps to contain the overall size. It is fine to cut it back now or in early spring before it starts to leaf out.

This hardy shrub can handle just about anything. They aren't picky on when and how much you prune. I prefer doing most pruning in winter, though. This will ensure that it get its best looking growth when it comes back. You can prune as much as you want, but i usually prune it to about 1 or 2 feet from the ground. Here is an article that will help: https://www.gardeningknowhow.com/ornamental/shrubs/butterfly-bush/butterfly-bush-pruning-how-to-prune-a-butterfly-bush.htm