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Yes, you can prune a potted naval orange tree before bringing it inside for the winter. Trim away any dead or overgrown branches to promote healthy growth. Once indoors, place the tree near a sunny window, ideally receiving 8 hours of sunlight daily. Water it when the top inch of soil feels dry, but avoid overwatering. Maintain a consistent indoor temperature between 55-70°F (13-21°C) and consider using a humidifier, as indoor heating can dry the air. Fertilize sparingly during the winter, and monitor for pests. Pruning and these simple care steps will help your potted naval orange tree thrive during the colder months indoors.

https://www.gardeningknowhow.com/edible/fruits/oranges/navel-orange.htm

https://www.gardeningknowhow.com/edible/fruits/oranges

Your tree could be getting too much sun and not enough water. Curled leaves are also caused by insects, disease, and other environmental conditions. This article should help pinpoint the problem:

https://www.gardeningknowhow.com/edible/fruits/oranges/leaf-curl-in-orange-trees.htm

If you have added anything to them recently, then it might be responding well to the treatment. If it was stressed and just now recovering, then this would indicate healthy growth. The same happens with my indoor citrus after having to cure a round of spider mites. Once it recovers it will start to grow much larger growth.

As for the bitterness- There are several causes to consider. If they cross pollinated, then this years fruit will not be affected. This will only show up in then next generation after starting new seeds from one of the parents. What could be happening, though, is usually environmental. This can indicate pH issues or nutrient needs, as well as soil quality problems. Sometimes specific cultivars can be more bitter.

https://www.gardeningknowhow.com/edible/fruits/calamondins/growing-calamondin-trees.htm

https://www.gardeningknowhow.com/edible/fruits/oranges/growing-an-orange-tree.htm

They are water sprouts on the tree. You wouldn't want branches that low. Go ahead and pinch those off. Here is more:

https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-tree-pruning.htm

Yes, cutting the top back will encourage side branches. These articles should help:

https://www.gardeningknowhow.com/ornamental/shrubs/shgen/heading-cuts-in-pruning.htm

https://www.gardeningknowhow.com/edible/fruits/oranges/growing-an-orange-tree.htm

You will need to check the label of the insecticide you used. It says something like fruit and citrus insect control. It should say whether it is safe for edibles or if you must wait a certain length of time before consuming the edible.

If it was an insecticidal soap you will be Ok to consume the oranges.

https://www.gardeningknowhow.com/edible/fruits/fegen/fruit-tree-spraying.htm