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How deep was the mulch? More than 3-4 inches can smother a plant. Did you put mulch around the trunk?

Possible it's weather related or lack of pollination. This article might help; https://www.gardeningknowhow.com/edible/fruits/apples/no-fruit-on-apple-trees.htm

Yes, you should remove the rootstock growth. It does rob the graft of nutrients and will eventually kill it, if left to grow.

There's a big difference between moss and mold. Moss can and does grow on most trees, and is harmless. Mold is usually a sign of some disease or damage on the tree. It sounds like you could use the help of a professional to see if you have a problem, and what to do about it. You can contact a registered arborist in your area, or you can call the nearest botanical garden who will either advise you, or connect you with the local master gardener program.

Here is a link to proper care of your Mandarin tree.

To much water may be your issue, but inspect your plant carefully for any insect damage or disease.

https://www.gardeningknowhow.com/edible/fruits/tangerine/mandarin-orange-tree-care.htm

How old is this tree?

The thorns are not necessarily a red flag. Many young citrus trees develop thorns to protect their delicate vegetation. As the tree matures, it should grow thorns less frequently. This article will explain the thorns in more detail:
https://www.gardeningknowhow.com/edible/fruits/citrus/thorns-on-citrus-trees.htm

Is this the first time this tree has started fruiting? If no, has it ever fruited successfully? If yes, then it may be too early to judge on the fruit. If the fruit, after full development and ripening, turns out abnormally then you may want to consider that the rootstock may have overtaken the scion. Almost all citrus trees sold are grafted trees. The desired tree is grafted onto a very hardy but inferior fruit tree. This insures the superior fruit have a strong root system. But, sometimes, mostly due to damage to the scion (the good tree), the rootstock takes over. Once the rootstock takes over, the only thing to do is to remove the tree and start over.

This article will explain more:
https://www.gardeningknowhow.com/ornamental/trees/tgen/can-grafted-trees-revert-to-their-root-stock.htm