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This is a Medinilla plant.

https://www.gardeningknowhow.com/houseplants/medinilla/caring-for-medinilla-plants.htm

Replant in a rich but light soil. Mix equal parts composted bark and coarse peat, add a small amount of composted manure. Standard potting soil should be avoided. Orchid soil can be used, preferably amended with some composted manure.

https://www.gardeningknowhow.com/houseplants/medinilla/caring-for-medinilla-plants.htm

Harvesting seeds usually means letting the flower pods swell and dry before. The process is the same for most plants.

I will include two articles that you will find helpful:

https://www.gardeningknowhow.com/ornamental/flowers/fgen/harvesting-garden-seeds.htm

https://www.gardeningknowhow.com/houseplants/medinilla/caring-for-medinilla-plants.htm

It will help me to see the damage. Usually, peroxide will not cause this kind of damage at that strength. This was regular 3% wasn't it? These gnats are actually very beneficial to your plant. They eat, only, rotting material in the soil. If there were so many that you began noticing them, then this is a sign that the plant was already suffering from an infection in the roots. They will only usually fly around if the soil remains too wet for long.

It is quite possible that they were keeping an infection at bay from overwatering, and removing them allowed the infection to take over. The peroxide should help this recover. Just make sure to let the soil dry out on top before watering. This will keep the gnats in check as well.

Medinilla plants are best propagated by divisions, this article may help:

http://www.gardeningknowhow.com/houseplants/medinilla/caring-for-medinilla-plants.htm

This article will help you in the care of these plants:https://www.gardeningknowhow.com/houseplants/medinilla/caring-for-medinilla-plants.htm