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You will want to determine why your tree's leaves dropped.
Is the tree stressed; environmental issues, watering, pests or disease?
Are there markings on the leaves?
These articles may help you pinpoint the issue.

https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-anthracnose-treatment.htm
https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-algal-leaf-disease.htm

Keep the larger branches intact, and trim any smaller branches that cross the path of any other branches. This article will help: https://www.gardeningknowhow.com/edible/fruits/avocado/pruning-avocado-trees.htm

This could be due to pests or disease; from your description I might suggest Cercospora Leaf Spot.
These articles have more information.
I would also suggest taking leaf samples to your County Extension Office, they may help with a diagnosis more specific to your region.
I have listed a link to help you locate your nearest office.

https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-pests-and-diseases.htm

https://www.daf.qld.gov.au/plants/fruit-and-vegetables/a-z-list-of-horticultural-diseases-and-disorders/cercospora-leaf-spot

https://www.gardeningknowhow.com/extension-search/

You can save this tree, but it will change the shape of it, now that it is suffering and has lost some growth. This isn't necessarily bad though. You will have to apply both wettable sulfur, and dolomitic lime to the soil. This will kill of any infection from over-watering. Give this tree as much light as you can. Let the top two inches of the soil dry out completely before you water again, as they need a little bit of a dry spell between waterings.https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

You can grow them from seed, but it is difficult in container. Very difficult to keep it alive long enough to bear flowers. This could take anywhere near ten years. Now let's address the fruiting habits: These trees are dioecious, meaning that there are separate sexes. The male produces the pollen, and the female produces the fruit if it receives pollen. So you will have to have at least two, and hope that one is a female. You wouldn't know for a decade. The more resonable proposition would be to germinate 10 or so seeds. This would be a literal forest that you would have to bring inside for the winter.

The most appropriate action here would be to purchase a dwarf variety that is suited for container from a source that already knows the tree is female. You can then graft a male branch on the tree and it will, then bear fruit.

Long story short... No, one tree indoors will never bear fruit. But they do leech toxins out of the air indoors.

This appears to be a fungal issue. I would apply wettable sulfur around the area, and prune off any dead branches. Here is a link to an article that will help: https://www.gardeningknowhow.com/edible/fruits/avocado/