Just don't repot it!
Oh, too late?
Well then... Water it deeply and fully and keep it in partial shade. Don't "baby" it and try to change things up too much. Trees in pots need the air exchange we provide by watering to "flush out" old stagnant air and any nastiness growing in it. They also need to get established well in a pot before being moved around a bunch or repotted...
I SERIOUSLY doubt over watering was the cause of your issues unless the tree was sitting in a pot without any drainage holes or in a POND. More likely it got too dry or had some sort of soil issue like over fertilizing that burns the roots and leads to the dead root mass you are describing... That can in turn lead to soil that doesn't dry out because roots aren't taking up the water... But it doesn't mean over watering was the culprit!
Regardless, treat for potential insect/ fungus damage, set it aside in partial shade, WATER IT HEAVILY when you do water it- til water runs through the pot, wait, then do it again. This flushes the old stagnant air out of the soil like I said. Don't try to "mist it" or water sparingly. If the tree has any life left it might come back, or it will turn brown and into firewood... But if you keep messing with it, pulling it out of the pot, poking at it, fawning over it, trying to do "tricks", it will die. Guaranteed. That IS a the "trick"- stick em in dirt, water them often, besides that forget all about them!
Good luck.
