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What have the temperatures been during the recent Fall nights?
Hard to see but looks like tbat graft union was very close to the soil or possibly underground at sone point. This is extremely bad and can kill a tree. Mangoes should be planted a tad "high", and in any event the graft should definitely be above the soil line and mulch.
When I saw your initial photos of the leaf petioles, my first thought was "fungus". Your description of gray/raisin weakening adds to my suspicion of fungus.I see you have already moved the plant indoors where is it likely to be less humid. This should help and I hope it does. Good luck!