Lemme try to explain this to myself:
There's a Being. His heart gets hit with an impossibly large amount of light. It overcomes his heart. The exact opposite case of a Heartless. We'll use algebra.
X = -Y
X being Garforge and Y being a Heartless. A Being's heart being so pumped full of light that all the darkness in it is eclipsed. The heart falls into a complete state of light. The Being can still think and act, as they still exist as a Being would. They just defy, or rather are unaffected, by all the natural laws that effect Beings with darkness in their hearts.
That right so far?
Yeszir.
Lol so how does one with a heart born from the darkness, invert, fleet to the light then return. Who's to say he couldn't die if he fell to the light? He really could've, but salvation befell him and to misunderstand in the way this was delivered leads to what will probably be his own end. I'll say this much.
As long as he moves without a shadow, the heart principles as you said hold little to no binding, but I didn't say there wasn't already a way for the shadow to return, Shad already knew in hindsight.