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If they have brown edges, then the tree is not getting enough water. Newly planted trees need LOTS of water to establish well. You almost cannot overwater them the first year or so (but you don't want them to be in standing water) Increase water to the tree and this should help.

This indicates a watering issue. Either the tree is not getting enough water or there is something affecting the roots that is preventing them from taking up water.

You can do some judicious pruning, no more than 1/3 of the branches at a time. The 2 small ones can be turned into houseplants if you chose or transplant to another location. In order to bear fruit they avocado trees need pollinators. You might want to consider planting flowers that draw bees and other similar pollinators to encourage your tree to bear more fruit.

Here is an article that you may find helpful: https://www.gardeningknowhow.com/edible/fruits/citrus/leaves-falling-off-citrus-tree.htm

If necessary, pruning may be done to maintain their shape and height. As for harvesting--Grapefruits are usually harvested in fall. Most lemon varieties have a long bearing period and can be harvested between August and February. Once the fruits have turned yellow or gold in color, they’re ready for picking. The longer the fruit remains on tree, however, the larger and sweeter it becomes. The most reliable method is the taste test. You can also squeeze the fruit with your hand: if it is really hard, it probably needs to stay on the tree longer.

Water avocados deeply once every 7-10 days.

Yes, unless you will be hand-pollinating the tree yourself. Avocado trees produce flowers that have separate male and female phases. For example, when the flowers open, they may be female one day and male the next. Type A trees are female in the morning and male in the afternoon. Type B trees open with female flowers in the afternoon but are male in the morning. You need the opposite type as a pollinator, or save the pollen one day and use it the next to pollinate the tree by hand.

Do not tar over the wound. This can trap bacteria and fungus in the wound, which will lead to rot. You should let it air dry.