I move in with a lonely poro to attack a battlefield controlled by my opponent. I let the combat resolve and my poro dies - can I then react to the Lonely Poro deathknell trigger and Ambush Nidalee down to the battlefield my Lonely Poro was attacking?

No, you can't play Nidalee there.

Two separate rules block this. First, Nidalee's Ambush says "to a battlefield where you have units" — your Lonely Poro just died, so you have no units at that battlefield anymore. Second, you don't control that battlefield mid-combat (control only transfers after Conquer resolves post-Combat, Rule 464.1.b), and Rule 813.3.a is clear: Reaction units still play only to your Base or a battlefield you control. You could play Nidalee to your Base or another battlefield you control while the Deathknell is on the chain, but not to the enemy's battlefield.