Saving Grace's ability has no effect on damage already dealt earlier in the turn.

If Saving Grace leaves the battlefield during the turn its triggered ability resolved, damage will continue to be redirected to the creature it enchanted before it left the battlefield. If the creature Saving Grace was last attached to isn't on the battlefield or isn't a creature at the time damage would be dealt, it won't be redirected.

More damage can be redirected to the enchanted creature than it has toughness, as long as that damage is all dealt at once (like combat damage is).

Saving Grace's redirection effect doesn't change the source of the damage or whether the damage is combat damage.

If you have more than one Saving Grace enter the battlefield in one turn, all damage that would be dealt at once to you and/or permanents you control is dealt to one of the enchanted creatures of your choice. It's not dealt to all of them, and you can't split the damage between them.

If noncombat damage would be dealt to a planeswalker you control, the planeswalker redirection effect and Saving Grace's redirection effect apply in whichever order you choose. However, no matter which order you choose to apply them in, that damage will be dealt to the enchanted creature instead.