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They will do this in response to too little water. As long as they have good drainage, it will not hurt them to get extra water. Just in case, it does not hurt to check for root rot (https://www.gardeningknowhow.com/plant-problems/disease/treating-root-rot-gardening-tips-for-housplants.htm).

It sounds like the tree has a pest, maybe scale or aphids. It is hard to say as there are several pests that cause this kind of damage. We recommend treating these kinds pest with neem oil. It is safe for humans and animals but is very effective against most destructive plant pests.

Citrus trees can take up to 6 years to mature and be able to bear fruit, especially when they are grown from seed. If you wait, then yes, it will eventually bloom and produce fruit.

These articles will help you with fertilizer:
https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/learn-about-fertilizer-for-a-lemon-tree.htm

This typically occurs due to a lack of water. I would increase water to the tree and it should regrow its leaves.

This normally indicates that the plant is not getting enough water. Try increasing the water. Also make sure that it is thoroughly watered. Water it until the water comes out the bottom to make sure it is reaching the bottom of the container.

Lemons are ripe and ready to pick once they have completely turned yellow. However, slightly green-yellow ones can still be picked and will normally ripen off the tree, provided they are large enough. They should also be somewhat heavy feeling and hard with a 'glossy' look to them. If it's kind of squishy, you've waited too long. The lemon should also be about 2-3 inches in diameter.