I cast Genesis Wave to get three cards. Those cards happen to be Eternal Witness, Mentor of the Meek, and Cloudstone Curio.
1) Can I get back Genesis Wave with Eternal Witness?
2) Will Mentor of the Meek allow me to draw a card because of Eternal Witness?
3) Will Cloudstone Curio trigger because of Mentor of the Meek to be able to return Eternal Witness back to my hand?
1. Yes. You don't put triggered abilities onto the stack and choose targets for them until the next time a player would receive priority, which happens after Genesis Wave finishes resolving and goes to your graveyard.
2. Yes. All permanents that enter the battlefield due to Genesis Wave's effect do so simultaneously, and will see each other enter the battlefield for the purpose of triggering abilities.
3. Yes, for the same reason as 2. Curio will also trigger due to Eternal Witness entering the battlefield.
1) Can I get back Genesis Wave with Eternal Witness?
2) Will Mentor of the Meek allow me to draw a card because of Eternal Witness?
3) Will Cloudstone Curio trigger because of Mentor of the Meek to be able to return Eternal Witness back to my hand?
2. Yes. All permanents that enter the battlefield due to Genesis Wave's effect do so simultaneously, and will see each other enter the battlefield for the purpose of triggering abilities.
3. Yes, for the same reason as 2. Curio will also trigger due to Eternal Witness entering the battlefield.