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Make sure you keep the soil moist at all times, not wet, just moist. Keep it away from cold, drafty windows and doors. I hope your plants are simply adjusting to a new environment and will settle down soon. I'm sorry to say that insects are often the cause of flower bud failure in hibiscus. Take a good hard look for tiny thrips or aphids. These can be difficult to eradicate pests in greenhouses. If your plants are infested, I'll bet T&M will replace them.

Since it is still doing well indoors, you may not even have to prune it unless you want to. I would wait until before it goes outside, unless this is strictly an indoor plant. If this is the case then you can prune anytime.

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

Black aphids do often attack hibiscus, so you're probably right that that's what they are. Here are some articles that will provide some control options:
https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm
http://extension.colostate.edu/topic-areas/insects/managing-houseplant-pests-5-595/
https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm

Hi Janny - Hibiscus trees struggle when they are abruptly brought indoors for the colder months. The reduction in light that occurs indoors is greater than most folks realize. But you can't fool Mother Nature and the plant tends to react by dropping a lot of leaves that emerged in outdoor sunlight. This is not fatal; just the plant's effort to adapt to the change in its environment.

Provide as much sunlight as possible. Water the soil as soon as it is almost dry on the surface. Prune the stems back sharply. You will see healthy new leaves emerging. However, they will be adapted to indoor light and will suffer when moved back outside in the warmer weather. That means you will need to prune it back sharply again just before moving it outside when it will thrive once again.

In colder climates, Hibiscus go through this difficult adjustment cycle twice every year when they are moved in and out. Some folks don't fuss about winter deterioration and just try to keep their Hibiscus alive through the winter in anticipatioin of warmer days.

Potted plants may need watering twice a day when temperatures are hot. Soil should be moist.

Yellowing leaves can indicate watering issues or pests.

These articles will help you.

https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/how-to-care-for-hibiscus-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/answers-to-what-causes-hibiscus-leaves-turning-yellow.htm

Pruning basics apply here. Any time of year, prune out dead, dying, diseased and crossing (rubbing) branches. Typically, hibiscus is pruned in the dormant season; growing a deciduous hibiscus indoors is going to throw this off. No matter; hibiscus is very hardy. Start with the 4 Ds listed above. Then prune out the oldest wood to soil level. Prune out any side branches that are growing toward the center of the plant. It should have a classic vase or V shape. Trim back the tips to the desired height, making the cut 1/4 inch above a node or set of leaves. If I'm right about what type of hibiscus you have, living indoors is not going to be good. Consider purchasing a tropical hibiscus. https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-container-care.htm