By the time Eternal witness enters the battlefield, will Collected company be already in the graveyard so I can put it back to my hand?
Yes, you can add Collected Company to your hand.
It works like this:
- Collected Company resolves choosing Eternal Witness
- Eternal Witness is put onto the battlefield and triggers (but is not put on the stack yet)
- Collected Company finishes resolving and is put into the graveyard as the last action of casting a spell
- State based actions are checked and see that Eternal Witness has triggered and it's ability is put onto the stack and targets are chosen
By the time Eternal Witness' ability is on the stack and it's time to choose targets, Collected Company is in the graveyard and can be chosen to be returned to hand