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You can top it. That will be fine. It is important to note that avocados will need a warm environment to survive. They will also need a second tree as male and female flowers grow on separate trees. You will need to know the sex of your tree to know which other tree you will need to get.

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

You will always have to have this tree in container. It will never be able to survive the winters in your area. It is also important to know that your tree will not show its sex for several years. You will need a second tree of the opposite sex in order to produce fruit, as male and female flowers form on separate trees and need both to pollinate.

Here is an article to help you with growing avocado: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

Here is one for indoor, specifically, container: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

It is hard to know without seeing the culprit... but by your description, it sounds like a spittlebug. I use neem oil for such applications. just don't spray when the sun is bright. overcast is the best time.

Here is a link to articles with more solutions: https://www.gardeningknowhow.com/special/organic/

The holes in the leaves could be caused by caterpillars, snails, or other pests. The best control method is usually to check the plant frequently and remove and kill any pests you see feeding on it.

The brown, crispy leaf edges are probably unrelated to the holes- they are usually related to either underwatering/ poor root system or to salt or fertilizer buildup in the soil. Please see this article for possible causes:
https://www.gardeningknowhow.com/plant-problems/environmental/what-causes-brown-edges-on-leaves-of-plant.htm

Also, make sure you are not weakening your tree by staking it. While trees may need staking under specific conditions, this should always be temporary (less than a year) and the tree should be able to sway with the wind, which strengthens the trunk over time:
http://www.myminnesotawoods.umn.edu/2008/12/staking-and-guying-trees-best-materials-and-technique/

Yes. Your tree will grow new leaves. Sometimes it helps to cut the tip off the force leaves. It is also important to note that avocados need quite a bit of light. Even a south facing window will not work in the winter. The lux count is half of that of the full summer outdoor sun, and is not adequate to sustain a mature tree. I would recommend about 200 watts of horticultural light through the winter.

After the temperatures outside remain above 60 degrees then it can safely go back outside.

When I don't have room under my light, as I produce veggies over the winter inside, Even in the brightest window, still loses most of its leaves through the winter. This is normal, and it will recover. Just remember to water less when it has less leaves. They won't be able to take it up as fast.

Here is an article for keeping avocados in container: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

it may have remained too wet for too long at some point. It appears to have an infection. You can try putting wettable sulfur and dolomitic lime in for the next time you water, and this can help kill off the infection. They really need more light than inside a house can give them. You might want to invest in a horticultural fixture of at least 100 watts.

This article will help you further: https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm