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Here is an article that will help you.

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

This is Scale.

You can treat with Neem Oil.

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/neem-oil-uses.htm
https://www.gardeningknowhow.com/plant-problems/pests/insects/control-plant-scale.htm
https://www.gardeningknowhow.com/edible/fruits/lemons/pests-of-lemon-trees.htm

Yes, you can do that if it's warm outside. Although many lemon trees can tolerate brief periods of temperatures as low as 26 degrees Fahrenheit, I wouldn't recommend that for a sapling. Introduce the tree gradually to outside conditions (a few hours of sun) each day and then bring it back indoors each night for a week. This helps to get the sapling used to being outside. You do this to avoid shocking the sapling and causing it to loose its leaves.

I would try starting the seeds in a smaller container.
This will help the seedling develop stronger roots and it is less likely to suffer from overwatering.
Here is a link with more information.
https://www.gardeningknowhow.com/edible/fruits/lemons/propagating-lemon-seeds.htm

In your case, I do not believe anything is wrong. Lemon trees typically need to be about 4 years old to fruit so you just need to wait a bit longer.

This is a common issue with citrus trees, wind blowing off blossoms.
You can provide your tree the optimum care.

Depending on the variety of Lemon Tree that you have planted, some cultivators bloom up to 4 times a year.

https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-blooming-season.htm