Yes, even on the healthiest plant, a leaf will die occasionally. More sun is good, tho. I don't think there's anything you can actually do to this plant to hurt it except rot the roots. In which case you could still probably continue the top growth because these plants have an interesting ability to make a "stopping point" on their stem. I don't know what the technical name is for this thing but I've seen it happen, the top part just falls off, looking for a more suitable place to start new roots. This also happens sometimes if the stem gets pinched against the edge of the pot. The weight of a stem can cause a break/stopping point at the rim of the pot. Just stick it back in.

its invasive in many areas.. and if in your area... get rid of it ... its potential is 10 feet tall and 12 wide.. over the years... so its improperly sited ... bugs had nothing to do with this.. this plant is nearly indestructible ... you could take a mature one.. and cut it to 3 inches.. and within a year or two... it would be bigger than it was before ... i suspect herbicide damage ... cut it to the ground.. and see what happens ... ken

Sorry, probably best to move it for the long term health of the tree. You can look into building it in a raised bed or something also. Mangoes have decent sized roots when mature so as far away from the canal as you can manage is better for it in the long run. How cold does it get where you live? You might be able to put a more salt-tolerant fruit tree in its place.

Generally speaking - and I'm no expert here, just have a few houseplants and have been learning a lot from this forum - some plants such as the softer leaves ones will show their displeasure and happiness quite quickly by wilting or perking up quite quickly. But I think some of the thicker leaved firmer plants can look ok for some time even when declining (more water reserves, stronger leaf structure,etc), for example sansevieria (snake plants), so maybe rubber plants are like this too. What I'm trying to say is it may have been going downhill for longer than the two weeks of obvious symptoms. Plus it's possible really high numbers of fungus gnat larvae could have munched on the really fine feeder roots. Although I thought the roots looked good overall. I don't know about the white bugs, you'd need to be sure what they were to know if they damaged it. And the (necessary) repotting was another stress on the plant. Many factors to consider, playing plant detective. It may well be the additive effect of several things. You've had lots of good advice from the experienced people above, on light and soils etc. Personally I would now put it in a bright warm draft-free spot inside, and wait. Don't let the roots dry right out while they're recovering but don't let the soil stay damp either. To achieve this, try using a spray bottle of water on a gentle jet to target where the pruned rootball is, and at the times when you do water more thoroughly leave the pot tilted at 45 degrees (even 30 degrees will help) for 15-20 minutes afterwards and then sit it on towels or paper pushed up against the drain holes for a couple hours. This isn't to remove all the water you just put in, it is to reduce the saturated layer that occurs at the bottom, the "perched water table" (yes, it has a name) Dont fertilise until you see new leaves growing and don't sunburn it by putting it in direct sunshine outside if it's not used to it. Good luck!