What happens if a player ativates Soulfire Grand Master, then casts Center Soul?

My understanding is that Rebound is not a self-replacement effect, so the player can choose which replacement effect applies, but I'd like an Official answer.

Your understanding is correct. Rebound does not create a self-replacement effect, so when Center Soul attempts to go to the graveyard, the game sees that there are two applicable replacement effects that could be applied, and asks the player to choose one of them to apply.

Applying either effect causes the remaining effect to no longer be applicable, and the spell is moved to whichever zone the player chose for it to go.