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A heavy pruning will often cause the flowering process to halt for awhile. Calcium nitrate will not likely help with flowering, as nitrogen usually prevents flowering in excess. The calcium can help with deficiencies, but it will no, necessarily, help with flowering. Bleach can help with ornamentals, but it can also take away a few years of its lifespan also.

I would recommend letting them fully recover from the hard pruning, then adding potassium and phosphorus. You can also try spraying with cytokines, but this can be dangerous to your health if misused.

If this is a dwarf variety, then go ahead. If this is a standard, then I would not do this. It will make it bushy, but branching it this low will make for a difficult tree to maintain. If this is where you want the branching to start, then you can prune, but next year you will want to take off almost all of the lower growth so that it does not become a tangled mess.

I would, personally, still wait one more season at least.

This article has information for you.

https://www.gardeningknowhow.com/edible/fruits/mango/mango-trees-in-pots.htm

Can you send a picture? This can be hard to diagnose, as this can happen for many, many reasons. If I am able to see it, then I can better diagnose what is going on.

I would imagine that a fungal infection has set in. Make sure to only water when the top of the soil is almost completely dry. At this point, if all leaves are turning brown, then it could be that it is passed the point of saving.

This article will give you more information on the care of these: https://www.gardeningknowhow.com/edible/fruits/mango/growing-mango-trees.htm

Most Mangos are ripe in June and July with a few varieties that ripen into August.

This looks like it could be sap burn.
These articles have more information.
https://www.gardeningknowhow.com/edible/fruits/mango/mango-fruit-harvest.htm
https://www.gardeningknowhow.com/edible/fruits/mango/mango-tree-not-producing.htm