[Squeek]

There's a flaw in your theory, Soujiro.

Harry's protection of everyone else was indeed the same protection his mother gave him. Willing to die to save others protects them.

Here's your flaw: his mother actually died.

Why then, did Harry live? Was it because Voldemort only killed the piece of his own soul instead of Harry, or was it because he was the Master of Death?

The latter is easier to believe, but both are plausible. Why do I believe the latter? Because he had the right to choose whether to die or to live at the King's Cross.