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The most likely causes of the yellow leaves are low light levels, overwatering, or both. If the tree has been brought indoors recently, it is normal for it to lose some leaves as it adjusts to the lower indoor light levels. It will also need less water when it is indoors.

If that's not the issue, here are some other possibilities to check:
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaf-drop.htm
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

Your lemon tree could be getting too much water or could have a disease that's causing the leaves to yellow. These articles may help you:

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-not-fruiting.htm

and

https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm

and

https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaf-drop.htm

This is quite normal. This can take up to 9 months to turn yellow. The size is a very good sign. This indicates that the plant is well cared for. There is no cause for concern. Think of it as a growing baby, and care for it to the best of your ability.

I can tell you from experience, that container citrus are needy in terms of care. You will want to keep dolomitic lime and iron sulfate on hand at all times. This will prevent many problems that you will, inevitably, face while growing this tree. Use these at least once per year, or twice for good measure. You will also want to fertilize with a citrus fertilizer once per year for good measure, and fruit production. Keep the tree from freezing, or cold temperatures while the fruit is forming, and you will be rewarded with a wonderful crop.

Here is an article for further reading: https://www.gardeningknowhow.com/edible/fruits/lemons/growing-lemon-trees-containers.htm

It would appear that they can live for about 50 years. Yours should still have a little while left. It is best to fertilize them at least once per year. This could be the problem, if this isn't done once a year.

Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/lemons/how-to-grow-a-lemon-tree.htm

If you can, contact the arborist and ask for the name of the disease the trees are infected with. The best treatment will depend on what disease it is.
Armillaria root rot and Phytophthora root rot are some of the fungal and fungal-like diseases of orange trees. See the following page for images and links to more information about treating these diseases:
http://ipm.ucanr.edu/PMG/C107/m107bpleaftwigdis.html

Also see this page for more fungal diseases of citrus:
http://cals.arizona.edu/pubs/diseases/az1154/

This article will help you find the cause of the dropping leaves:
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-leaf-drop.htm

I am not sure what could cause the soil to turn yellow- perhaps it's a mold that could be scraped off?