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Sometimes this is normal. A photo will help me to see what is going on, though. I would transplant ,this to soil immediately. This is about as long as you want to leave it in water. There are not enough nutrients in it to sustain growth. As the tree depletes the nutrition stored in the seed shell, it will require soil to keep it alive.

This article will help: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

I believe you would need to start this process on your end.

I found this link for you.
http://www.inspection.gc.ca/plants/exports/eng/1299164931224/1299165527958

Pruning can be done after your fruit have ripened, usually mid-summer.
These articles will help you.

https://www.gardeningknowhow.com/edible/fruits/figs/potted-fig-tree-pruning.htm
https://www.gardeningknowhow.com/edible/fruits/figs/pruning-fig-trees.htm

Is this in container? This looks like a severe magnesium deficiency... Dolomitic lime will correct this. This article will help: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

This articles should help

http://www.gardeningknowhow.com/plant-problems/pests/insects/control-whiteflies-houseplants.htm

It will not harm your tree in any way. Leaving in that water much longer will. You will need to plant this as soon as possible.

I, actually, prefer to top mine at 2 or 3 feet tall to make it easier to control as it branches out. You can cut it that short, but it will be very hard to manage later on.

This article will give you information on the care of these: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm
As will this one: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm