That's a lot of really good information. Thank you.
My first thought was you had a sucker from the roots. If that was the case, the leaves on the sucker would look different. I have a Kinnow mandarin (its my favorite citrus) - your tree looks exactly like a Kinno. Don't bother taking leaf photos.
With your added details and history, i have a couple thoughts but proving them will take a year.
I suspect a combination of:
1. A nutrient problem. Your gardener used the wrong fertilizer, removed it and didn't use anything else? Has your tree been fertilized in the last 3 years? You should be using citrus specific fertilizer every other month while the trees are in active growth. Citrus are heavy feeders and also really picky about what they require.
2. Root damage. Citrus have a shallow root system that's easily damaged by digging near it. Your gardener may have damaged the root system (more on one side of the tree than the other) and the tree is still recovering. What do you mean when you say you have isolated the ground between this tree and the sweet orange? Is that tree on the not producing fruit side of this tree? You may be continuing to damage the roots.
Insufficient fertilizer combined with damaged roots could easily cause parts of a tree to stop producing and even die. Even with proper fertilizer, if there aren't enough roots to absorb nutrients and water, its going to do poorly. Only time will tell... but in the mean time use the right fertilzer (next spring when new leaves start to grow) but use at about half the recommended rate until you see improvement in the overall tree health.
