MTGNexus

Collision of Realms

If I were to cast this card while a player controlled only their commander on their board as their sole creature, would that player put a creature onto the battlefield from their library as part of the second effect of Collision of Realms, assuming they use the commander replacement effect to return it to the command zone as it attempts to go to their library? I'm also assuming the same is true of most other relevant replacement effects, like Gruesome Encore?

No. Due to the condition of "Each player who shuffled a nontoken creature" it will look at who actually shuffled something into their library. With the Commander replacement effect and something like Gruesome Encore, the answer it gets is "none" since they didn't actually shuffle anything into the library. They will not get a free creature.