Update!
Alea iacta est, the decision is taken!
I couldn't stay inert watching my pomelo suffering.
It's risky to remove it from the soil but i had to do it to check what's going on down there.
So this afternoon taking advantage of a cloudy and "fresh" day i dug around and removed it from the soil
I was quite shocked to find thousands of tiny roots invading all the soil and for sure NOT belonging to the pomelo (brown color)
They are extremely thin and forming a kind of root sponge, evidently to suck any possible water and nutrient from the soil.
I have no idea where they're coming from since, apart from a young coconut (15 feet far and with completely differnet roots) the closest trees are a caimito and a manzanitas (muntingia calabura) but they are more than 10 meters (30 feet ) far.
Manzanitas id sadly famous for being a greedy sucker with its tiny and invading feeding roots but i can't imagine how they reached the pomelo since in the middle there is NO soil but only hard limestone with some bermuda grass planted on 1 inch of topsoil. I must find a solution to prevent them to grow again
Examining pomelo's root i could easily notice the difference between the new growth (white color) and the old rootball (yellowish color)
Millet you were so right! the pomelo was definitely rootbound. Only a few new roots developped and managed to spread around but the lower part is just a thick tangle
now i temporarily transferred the pomelo in a pot without touching the roots to avoid further stress, i watered it and i put the pot in a shadowed area.
Tomorrow I will clean the soil removing all the alien roots then i'm planning to loosen pomelo's roots trimming and spreading them in order to gve them a possibility to grow normally.
Let's see what will happen, further updates to come!