It does sound like you might have used too much fertilizer...I've used those fertilizers on mine but I only use about 1/3 of the recommended dosage (I only use 1 tsp instead of 1 TB) especially since you already have Osmocote in the mix. Have you checked the roots to see if it has started to rot? What type of potting mix is it in? Maybe its not draining properly in conjunction with too much fertilizer?

Also keep in mind that inground trees can handle a little more fertilizer than Potted trees...either way Mango trees are susceptible to fertilizer burn so use a lot less than what is recommended (very weak) is best.

I also don't fertilize when the tree is blooming. Good luck...hopefully it recovers for you...I'm sure you've stopped fertilizing...if not make sure you don't add anything else...only water when the soil is almost dry.

Maybe others have other opinions...

Maybe do a PH test of the soil in the pot. I'm wondering if the ph has become too high for some reason then it renders many minerals unavailable to the plant, and its minerals reserves run down.

I agree with Puglvr1, I think its very likely you may have over-fertilized. I killed a few of mangoes doing so. It wasn't until I purchased a Hanna EC meter that I was able to see how much fertilizer salts (over 3000 micro-seimens) were in my container. I flushed all my trees with about 5 gallons of water until the EC readings were at an acceptable range (between 200 & 1200 micro-seimens). I managed to save my Pickering (but I had to mega-pug it) and Lancetilla but my Southern Blush died. Fortunately, my trees are in fast draining media.
Also to gomango's point about Ph. I did discover that my local water had a Ph of 8.1 so I had to remedy it with citric acid to get it down to 6.5
good luck.

Invar
I have a similar micro-seimens meter from making colloidal silver. I have an HM Com-100. Do you have any more info on measuring fertilizer salts this way?

James Hoon -- when a fruit tree is in a pot you dole out fertilizer at the rate of 1-2 tablespoons per month except the winter months. Obviously you don't use as much as you do if the tree is planted in the ground. (I know I am saying nothing new here)

Zands - I'll have go through my saved links on one of my computers and see if I can find the original web page. The recommendation of 200-1200 microseimens came from a general recommended value for "trees." Here is a link to a pdf (it has a conversion table too) that lists various tolerable levels for salts.
I have to say my Hanna meter totally saved my mangoes and taught me how carelessly enthusiastic I was with my fertilizer usage. Once I got the EC readings to the above levels with flushing, my little trees almost immediately started putting out new growth.

Here is a link that might be useful: Testing soil

found the link...

Here is a link that might be useful: The why and how to testing the Electrical Conductivity of Soils

Invar---
Thanks for all that information on measuring for fertilizer residues (salts)