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Yes, as long as the trees were grafted ones purchased at a nursery or garden center, then you should be able to get fruit from them. If you are seeing lots of foliage growth with little to no blooming, then it's likely a nutrient imbalance - too much nitrogen, which produces lush growth and too little phosphorus, which is responsible for flowering and fruiting. Without flowers, you won't get fruit. I would give the trees a high phosphorus fertilizer, or add bone meal to the soil. Here is more information: https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/phosphorus-plant-growth.htm

No, but why would you want to? If you're planning to keep it as in indoor ornamental, 2 trunks would seem to add to its appearance. Here's more information on pruning your little tree: https://www.gardeningknowhow.com/edible/fruits/avocado/pruning-avocado-trees.htm

The most important thing is to have good drainage in the soil. Try potting soil or commercial "soilless" mix, and add perlite, 1 part perlite to 3 parts soil. Fertilize monthly with any balanced water soluble fertilizer with minor elements. This article has more information:https://www.gardeningknowhow.com/edible/fruits/avocado/indoor-avocado-plant-care.htm

This is a question about which there is still scientific debate. This article might shed some light for you: http://www.wildflower.org/expert/show.php?id=9536
If you have doubts, the only sure way is to have the soil tested. Dig up samples from several spots around where the oleander was, you can then mix them together for testing. Your local Extension Service should be able to test for you, or tell you where to send the sample. Explain to them that you're concerned about the presence of Oleandrin in the soil. This link will help you locate an office: https://www.gardeningknowhow.com/extension-search/

There's nothing we can do to stop the wind and rain, all we can do is to take are of the trees as best we can to make them as strong and healthy as possible, and after that it's up to mother nature. A 10' tree is not that old, so yours should make more flowers and fruit as it matures, and hopefully they will survive the storms. Here's some information to apply to growing a strong and healthy tree: https://www.gardeningknowhow.com/edible/fruits/avocado/avocado-tree-growing.htm

If you potted up from a 1 gal container to a 5 gal container, the new container is really too big. This means that the soil is staying too wet, and the roots are being slowly drowned. I think if you can take your avocado out of the 5 gal pot and put it into a smaller one then proceed as this article suggests, you may be able to save it: https://www.gardeningknowhow.com/houseplants/hpgen/repotting-stress-treatment.htm