I did see aphids on one of the yellow leaves in the picture. Aphids would also account for the curling of the leaves. There are winged aphids that usually take a hike when the plant they are on is stressed. They go out and look for a richer feeding ground. The flies do look more like flies. Winged aphids will have two spikes poking out of their rear ends and they have parallel wings. At first I thought the flies were some kind of fruit fly. Fruit flies usually roost on a host and only come to the trees that have fruit on them to lay eggs. After the larvae feeds on the fruit core it rots and falls to the ground and the larvae pupate in the soil. The pupa in winter can delay emergence over a few months. If this tree had some fallen fruit that was not picked up right a way, the flies may be emerging from the soil.
This is a stressed tree, the aphids feeding does not help. To get rid of the aphids, set out ant traps. The leaves are falling and mine only does that under stress like when the vines grow over it. I don't keep citrus indoors so I don't know it it is normal for it to go dormant in your area.
The aphids can be washed off with a sponge or cotton dipped in alcohol. It is a small plant so it can be done by hand. Make sure to inspect the stem as well as the leaves. You may have to do this every day until you don't see any more.
If the flies are emerging from the soil, I would put a layer of gravel on top of the soil to keep them from emerging. Make sure that the citrus is not being overwatered and empty the saucers so there is no standing water. People tend to water their plants more when they are in distress and over watering kills more plants than under watering. Learn how to test when your soil needs water with a chopstick or your finger or get a water meter. It really helps with indoor plants. Put a fan near the area and keep it running on a low setting during the day. Air circulation keeps the bugs away and helps to dry things out better.
Something like this really helps when you are gardening indoors. Plants are stressed indoors since they don't often get enough light or air circulation and people water the pots in place and the saucers are not emptied afterwards. This can weaken the plants and make them targets for bugs. Overwatering attracts fungus gnats as well. Indoors with poor air circulation and saucers under plants, the media does not dry very well, stressed and dormant plants need less water than actively growing ones. It is important to know when to water. Taking the plants out or to a tub at least and hosing the dust off the leaves helps get rid of any bugs on the leaves and more light can get to the leaves if they don't have dust on them. Salts will accumulate in a pot and be detrimental to the plant so it is a good idea to flush the pot with water not just put a cup of water in the pot when you do water.
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Happy gardening in Hawaii. Gardens are where people grow.