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This could either be attributed to transplant shock, which normally corrects itself once the plant adjusts to the new surroundings, OR it could be a fungal issue, which can be taken care of with fungicide. Check that the tree has suitable drainage and is not remaining too wet, which can often lead to fungal problems. While it is important that newly planted trees receive plenty of moisture to establish roots, they do not need to become waterlogged.

We normally recommend treating with neem oil. Here is more information: https://www.gardeningknowhow.com/problems/pests/neem-oil-uses.htm

It is certainly possible that the weed and feed could have affected your tangerine, though not likely because the tree is so much larger than the grass. There are a couple of questions you might ask yourself -- was the tree all right until the chemical was applied? how long have you had it and how much has it been fruiting for you? Here is an article that might give you some food for thought: http://www.crec.ifas.ufl.edu/academics/classes/hos6546/materials/Smith.pdf
You might want to have the soil and leaves/branches/fruit analyzed by the Extension Service. (This link will help you locate a nearby one: https://www.gardeningknowhow.com/extension-search/
You might also take in some of the weedandfeed, if you have any left. If there's a product out there with something wrong with it, the state might want to know.

This normally happens due to watering issues. The best thing you can do is just make sure it gets the water it needs. The leaves will grow back. Here is more information: https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm

Start by checking the care your tree is getting: https://www.gardeningknowhow.com/edible/fruits/tangerine/tangerine-tree-care.htm
If you don't see anything here that rings bells, this article on orange trees that flower but don't fruit is also applicable to tangerine: https://www.gardeningknowhow.com/edible/fruits/oranges/no-fruit-on-orange-trees.htm

This article will take you through the steps to having a healthy tangerine tree: https://www.gardeningknowhow.com/edible/fruits/tangerine/tangerine-tree-care.htm
Trees in their nursery pots frequently fruit. When you plant it, it will probably a couple of years before it starts bearing much fruit.