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A healthy Calamondin tree can live 40 or more years.
This lifespan can certainly be cut back by disease, pests or environmental issues.
This sounds to be quite a large sized specimen and pruning dead plant material may be best preformed by a professional.
A good inspection of the tree by an Arborist, can help determine if the tree is suffering from an infestation, fungus or nutritional problems.

Here are a few articles with more information.

https://www.gardeningknowhow.com/edible/fruits/calamondins/growing-calamondin-trees.htm
https://aggie-horticulture.tamu.edu/patiocitrus/Calamondin.html
https://www.gardeningknowhow.com/edible/fruits/oranges/pruning-orange-trees.htm
https://www.gardeningknowhow.com/edible/fruits/citrus/

Calamondin is an edible citrus fruit.

This is common to seed this type of label put on citrus trees from the grower.
Numerous chemicals are applied to the plants in greenhouse production.
If the tree has fruit now, it is best to remove those and Not consume those.
The new fruit that grows while in your care, would be safe to eat.

Here is a link with care information.

https://www.gardeningknowhow.com/edible/fruits/calamondins/growing-calamondin-trees.htm

Calamondin grown from seed should produce fruit after it's 2 years old. The fruit may not be true to seed.
I would look at amount of sunlight, watering and nutrients and make adjustments.
The links below will help refresh you on the care requirements.

https://aggie-horticulture.tamu.edu/patiocitrus/Calamondin.html
https://www.gardeningknowhow.com/edible/fruits/calamondins/growing-calamondin-trees.htm

This could be a difference between individual trees based on genetics, or it could be because of small differences in environment that the trees have experienced while growing. Citrus are quite variable in whether they produce thorns or not. Here is more about citrus thorns:

https://www.gardeningknowhow.com/edible/fruits/citrus/thorns-on-citrus-trees.htm

Yes, a grow light or two are usually necessary for citrus trees to produce fruit and stay healthy indoors. Indoor light is almost always dimmer than outdoor light, so most plants will need an adjustment period when brought indoors, especially sun-loving plants like citrus. The more light you can give it (within reason), the better it will look. Also, decrease watering, because the tree will use less water while indoors.

Here are more considerations for citrus care indoors:
https://www.gardeningknowhow.com/edible/fruits/citrus/growing-citrus-trees-indoors.htm

You most likely have a branch growing from the root stock.
The Calamondin is grafted onto the hardy root stock of a citrus, which may be a sour orange variety.
Suckers that grow from the root stock should be pruned away as soon as they appear.

https://www.gardeningknowhow.com/edible/fruits/calamondins/growing-calamondin-trees.htm