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There are a few common pests on lemon trees and citrus.
This article will help you.

https://www.gardeningknowhow.com/edible/fruits/lemons/pests-of-lemon-trees.htm

They are suffering from a lack of light! These require more light than is available in most homes. Unless you have a south facing window, where they get sunlight all day unobstructed, then they won't do well without supplemental light.

Generally, 100 watts per tree will keep it alive, and 200 will allow it to fruit, as long as it is maintained for size. Otherwise, they will lose their leaves in the winter, even indoors. This is a semi dormant state to protect itself from less than ideal growing conditions.

As long as danger of frost has passed and night temperatures are not below 40 to 50 degrees F. you can move your plant outdoors. Make sure you transition it to full sun. Start with morning sunlight and then more shaded in the afternoon. Gradually work up to full sunlight.

Yes, but you will need to get more light, such as a 100 watt panel or better. It will also need to dry out more between waterings. At least 2 or 3 inches down should be COMPLETELY DRY before watering again.

Correct these two things, and you will see a very fast recovery. Aside from that, correct fertilization will be another key factor. This article will give you more information on their care: https://www.gardeningknowhow.com/edible/fruits/citrus/growing-citrus-trees-indoors.htm

Uh oh... This is a mealybug infestation. This insect will destroy your trees. You are going to need some really strong stuff to get rid of these. I don't normally recommend products on here, but in this case, I will. There is a product from FoxFarm called Force of Nature Miticide. It comes in a ready to use spray, or a concentrate if you would rather mix your own.

You will have to use this once every few days, and scrape as many off as you can, or cut the leaves. This is a massive problem, and if not handled immediately will end those trees.

This article will give you more information: https://www.gardeningknowhow.com/plant-problems/pests/insects/mealybugs-control.htm

Spraying during the heat or sun can cause some spots on the leaves, but without any image to go by I can't determine if this is the cause.
Some diseases could also cause yellow spots on leaves.
https://www.gardeningknowhow.com/edible/fruits/lemons/common-lemon-tree-diseases.htm