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Hopefully, these two articles will help you: https://www.gardeningknowhow.com/ornamental/trees/tgen/how-to-fix-girdled-trees.htm

https://www.gardeningknowhow.com/ornamental/trees/tgen/repairing-tree-bark-damage.htm

I doubt it was an animal such as rat, rabbit..squirrels are another thing, they climb and eat foliage/bark..
Are you near a body of water? Could it be beavers..they're quite destructive. What happened to your tree sounds like something they'd do..I've seen trees at the woods devoured by beavers.
I would suggest taking samples of the chewed bark and/or stems to your local agricultural extension office for identification and recommendations.

There are many reasons a plants leaves may yellow and fall off. For citrus, the issue is commonly a watering issue. These articles will help:
https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm

I would recommend that you have the soil around the tree tested. Color loss can be linked to lack of micro nutrients in the soil, and if you do not regularly fertilize, then the soil can be lacking in some nutrients. A soil test will help you pinpoint what exactly the soil is lacking and amend it accordingly.

If you have removed as much as you can, then the remaining fruit can simply be left on the tree. The tree will usually abort old or unhealthy fruit that's been left on the tree from previous year's on its own.

I would have your soil tested. Yellowing of the leaves is typically either an iron or nitrogen deficiency. When was the last time you fertilized the tree? It may need nutrients. If you have not been fertilizing, start doing so and, regardless, have the soil tested to see if you have any nutrient deficiencies that need to be corrected. Here is more information on fertilizing citrus trees: https://www.gardeningknowhow.com/edible/fruits/citrus/fertilizing-citrus-trees-best-practices-for-citrus-fertilizing.htm In addition, it could be a watering issue, possibly not enough. Citrus plants require lots of water. This article should help: https://www.gardeningknowhow.com/edible/fruits/citrus/tip-on-water-requirements-for-citrus-trees.htm This article will have some other reasons for yellowing leaves: https://www.gardeningknowhow.com/plant-problems/environmental/plant-leaves-turn-yellow.htm