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This is likely a water issue. The soil may not be draining away the excess moisture and the tree may be dying from root rot.

Dig down into the soil and check the moisture level.
The soil should be moist but not soggy.

Your tree looks quite stressed.

If root rot has taken hold, your tree may not survive.
This article will refresh you on the care requirements.

https://www.gardeningknowhow.com/edible/fruits/grapefruit/growing-grapefruit.htm

Yes, you are correct.
You should remove any fruit that may be on or forming on your young Grapefruit Tree.
You want your new tree to spend it's energy settling in and growing strong roots.
You can strip the fruit for the first year or two.
https://www.gardeningknowhow.com/edible/fruits/grapefruit/growing-grapefruit.htm

It is important to remove all dead and fallen fruit from the ground.
The fallen fruit will attract insects and disease.

You can place the fallen fruit in the compost.

One possibility for the dark color is sooty mold. Please check this article to see if symptoms match:
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm

This article discusses possible problems leading to few or no fruit:
https://www.gardeningknowhow.com/edible/fruits/grapefruit/grapefruit-tree-not-producing.htm

The dark, wrinkled leaves could be citrus leaf curl or one of the diseases discussed below:
https://www.gardeningknowhow.com/edible/fruits/citrus/curling-citrus-leaves.htm
http://ipm.ucanr.edu/PMG/C107/m107bpleaftwigdis.html

I strongly recommend that you get a soil test. Thick rinds often indicate a lack of phosphorus intake and/or too much nitrogen intake. It's mainly the phosphorus that helps produce the blossoms and then the fruit. Read about it here on GardeningKnowHow: Why Citrus Fruit Get Thick Peels And Little Pulp https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-thick-rind.htm

The picture and the symptoms you describe look consistent with citrus greening, an imported disease that has become a major problem in citrus-growing regions. The disease causes mottling with lighter-green areas that cross leaf veins plus leaf and fruit drop, and the symptoms usually affect only one section of the tree.

There may be other possibilities, like nutrient toxicity if the tree has recently been fertilized, but the best thing to do is to contact a local extension agent to check your tree.

Here is some more information about citrus greening:
http://www.citrusalert.com/about-citrus-greening/
https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-greening-disease.htm
http://ipm.ucanr.edu/PMG/C107/m107bpleaftwigdis.html